How kind of you to consider this project.
Yes, you can use knits but you will want to stabilize them first.
You need to ask her how much can you cut up the clothing. Infant clothing can be pretty easy to incorporate into a quilt, but 4 year old is getting a bit big.
I found this overall description pretty good, it has a link in it particularly about baby clothes:
https://www.toocooltshirtquilts.com/...ssed-loved-one