These trees are 5-7 years old and 4-6 feet tall. Would they work as Bonsai?
I have seen where tree trunks are cropped to about a foot high would this work for these trees?
Thank you.

Yes and yes

Maples backbud easily when cut back hard. Basically any maple is good fot bonsai. It just need time.

Japanese maples are amongst the maples in the group of more suitable species. Particularly the one on the last picture has a good start for bonsai