Code to create a custom configuration section with child collections

app.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="CLGSettings" type="Config.CLGSettings, MyAssembly" />
  </configSections>
  <CLGSettings defaultEnvironment="Training">
    <Environments>
      <Environment name="Training" webServiceURL="https://www.example.com/training">
        <Accounts>
          <Account username="TL-train" password="Password" serviceId="TL" serviceName="Argyll &amp; Bute" />
          <Account username="TM-train" password="Password" serviceId="TM" serviceName="East &amp; West Dunbartonshire" />
          <Account username="TN-train" password="Password" serviceId="TN" serviceName="North Lanarkshire" />
          <Account username="TP-train" password="Password" serviceId="TP" serviceName="South Lanarkshire" />
        </Accounts>
      </Environment>
      <Environment name="Live" webServiceURL="https://www.example.com/live">
        <Accounts>
          <Account username="TL-prod" password="Password" serviceId="TL" serviceName="Argyll &amp; Bute" />
          <Account username="TM-prod" password="Password" serviceId="TM" serviceName="East &amp; West Dunbartonshire" />
          <Account username="TN-prod" password="Password" serviceId="TN" serviceName="North Lanarkshire" />
          <Account username="TP-prod" password="Password" serviceId="TP" serviceName="South Lanarkshire" />
        </Accounts>
      </Environment>
    </Environments>
  </CLGSettings>
</configuration>

BaseConfigurationElementCollection.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Config
{
    public abstract class BaseConfigurationElementCollection<TKey, TElement> 
        : ConfigurationElementCollection, IEnumerable<TElement> 
        where TElement : ConfigurationElement, new()
    {
        protected override ConfigurationElement CreateNewElement()
        {
            return new TElement();
        }

        protected override object GetElementKey(ConfigurationElement element)
        {
            return GetElementKey((TElement)element);
        }

        protected abstract TKey GetElementKey(TElement element);

        IEnumerator<TElement> IEnumerable<TElement>.GetEnumerator()
        {
            foreach (TElement element in this)
            {
                yield return element;
            }
        }

        public virtual void Add(TElement configurationElement)
        {
            BaseAdd(configurationElement);
        }

        public virtual void Remove(TKey key)
        {
            BaseRemove(key);
        }

        protected virtual TElement Get(TKey key)
        {
            return (TElement)BaseGet(key);
        }

        protected virtual TElement Get(int index)
        {
            return (TElement)BaseGet(index);
        }

        public TElement this[TKey key]
        {
            get { return Get(key); }
        }

        public TElement this[int index]
        {
            get { return Get(index); }
        }
    }
}

CLGSettings.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Configuration;
using System.Collections;

namespace Config
{
    public class CLGSettings : ConfigurationSection
    {
        private static CLGSettings _settings = ConfigurationManager.GetSection("CLGSettings") as CLGSettings;

        public static CLGSettings Settings
        {
            get
            {
                return _settings;
            }
        }

        [ConfigurationProperty("defaultEnvironment", IsRequired = true)]
        public string DefaultEnvironment
        {
            get { return (string)this["defaultEnvironment"]; }
            set { this["defaultEnvironment"] = value; }
        }

        [ConfigurationProperty("Environments", IsRequired = true)]
        [ConfigurationCollection(typeof(EnvironmentSettings), AddItemName = "Environment")]
        public EnvironmentSettings Environments
        {
            get { return (EnvironmentSettings)this["Environments"]; }
            set { this["Environments"] = value; }
        }
    }

    public class EnvironmentSettings : BaseConfigurationElementCollection<string, EnvironmentSetting>
    {
        protected override string GetElementKey(EnvironmentSetting element)
        {
            return element.Name;
        }
    }

    public class EnvironmentSetting : ConfigurationElement
    {
        [ConfigurationProperty("Accounts", IsRequired = true)]
        [ConfigurationCollection(typeof(AccountSettings), AddItemName = "Account")]
        public AccountSettings Accounts
        {
            get { return (AccountSettings)this["Accounts"]; }
            set { this["Accounts"] = value; }
        }

        [ConfigurationProperty("name", IsKey = true, IsRequired = true)]
        public string Name
        {
            get { return (string)this["name"]; }
            set { this["name"] = value; }
        }

        [ConfigurationProperty("webServiceURL", IsRequired = true)]
        public string WebServiceURL
        {
            get { return (string)this["webServiceURL"]; }
            set { this["webServiceURL"] = value; }
        }
    }

    public class AccountSettings : BaseConfigurationElementCollection<string, AccountSetting>
    {
        protected override string GetElementKey(AccountSetting element)
        {
            return element.Username;
        }
    }

    public class AccountSetting : ConfigurationElement
    {
        [ConfigurationProperty("username", IsKey=true, IsRequired=true)]
        public string Username
        {
            get { return (string)this["username"]; }
            set { this["username"] = value; }
        }

        [ConfigurationProperty("password", IsRequired = true)]
        public string Password
        {
            get { return (string)this["password"]; }
            set { this["password"] = value; }
        }

        [ConfigurationProperty("serviceName", IsRequired = true)]
        public string ServiceName
        {
            get { return (string)this["serviceName"]; }
            set { this["serviceName"] = value; }
        }

        [ConfigurationProperty("serviceId", IsRequired = true)]
        public string ServiceID
        {
            get { return (string)this["serviceId"]; }
            set { this["serviceId"] = value; }
        }
    }
}