An example of merging 3 git repos into 1, and preserving all the history.

git-repo-merging.sh

#setting the stage... creating 3 separate repos
mkdir animals; cd animals/; echo "Animals" > README; git init; git add .; git commit -m "Animals readme"; cd ..
mkdir dogs; cd dogs/; echo "Dogs" > README; git init; git add .; git commit -m "Dogs readme"; cd ..
mkdir cats; cd cats/; echo "Cats" > README; git init; git add .; git commit -m "Cats readme"; cd ..

#putting stuff in boxes for the move...
cd cats; mkdir cats #making /cats inside of cats, because in /animals we want /cats to be a subdir
git mv README cats #this line needs to me more appropriate to the contents of your actual child repo
git commit -m "Prepare cats for merge into animals"; cd ..

#the short version, for dogs...
cd dogs; mkdir dogs; git mv README dogs; git commit -m "Prepare dogs for merge into animals"; cd ..

#time to combine the repos. let git remotes do the work.
cd animals;
git remote add cats ../cats; git pull cats master; git remote rm cats
git remote add dogs ../dogs; git pull dogs master; git remote rm dogs

git log
#you should see all the history in here.
#kill the original dogs and cats at your leisure