smthing.rb

u = User.new(email: '[email protected]')
u.password = u.email
u.save

child_1 = Kid.create!(name: 'kid 1', desc: 'has_camp', user_id: u.id)
child_2 = Kid.create!(name: 'kid 2', desc: 'has_camp', user_id: u.id)
child_3 = Kid.create!(name: 'kid 3', desc: 'without camp', user_id: u.id)


camp = Camp.create!(name: 'first_camp', desc: 'none')

CampsKid.create!(camp_id: camp.id, kid_id: child_1.id)
CampsKid.create!(camp_id: camp.id, kid_id: child_2.id)

CampsKid.joins(:kid).where(kids: {user_id: u.id}).each do |c_kid|
  puts "#{c_kid.kid.name} -- #{c_kid.camp.name}"
end

test 2