if they are 3 5 and 7 , i would get yourself some hicory or oak (im just guesstimating with this..) , and cut it like they say on the dean torges one.
most places dont call their plain sawn lumber "plain sawn" , its just "lumber" . i think sam harper or someone on *********** did a beginners bow build-along kind of thing , very similar to dean torges , where he cuts backing strips from a plain sawn , and uses the other pieces for the belly. this is what i would do for a kids bow , especially if i had to make 3 or more.
what kind of tools / woodworking stuff do you have? clamps? table saw or bandsaw?
this all helps give us an idea.
there is also rattan , that i hear is durable and inexpensive.
basically if i was you , depending on what wood you have available , id make them bend through the handle bows , and use dowels for arrows for right now. if they really dig it , get something as close to indestructable or cheap as you can... i was out today hunting with bowmofo , and while just standing there takin in the scenery and whatnot , i grabbed a dried cattail stalk , cleaned it up , and cut a little half-nock into it , and surprisingly it flew very well with no fletch , randomly cut to whatever length , and only half a nock. i really expected it to jusut snap and fold in half.
theres lots of help here , we just need to know what youre workin with so we dont tell you to go buy a bandsaw...yet...