C.F.
I had twin boys and making their costumes were challenging. but I can offer some suggestions.
little jack horner should be easy, using a flannel shirt, overalls(jeans) and then you would just need a straw hat.
the same with Jack of Jack and Jill.
if you have books for the children, look in their for the image and try to match it.
btw, I am an RN!