I built a red oak flatbow of 48" long, I think it draws about 40 or 45# @24", but that thing scares the hell out of me. The tiller is clean and it's frettign all over the place. It stays on teh wall and does not visit the range.
I think you are going way way way too short on this one. Especially if you are new to making bows. 48" anything is a challenge, 48" red oak is like mission impossible.
For red oak I'd recommend the bow be at least as tall as the archer. I don't know if the child is currently drawing 26" or you're planning for it. A draw length of 26" would be about right for somone who's maybe 5' 6" or so. I think you need at least 60 or 62" to try and make that draw length, and honestly I think even thats more trouble than you need. 64" to 66" will make things alot easier, and it'll take less set, cast should still be just fine.
48" should probably be more appropriate for a person of 4' to 4' 6" tall.
I draw 25" and I make my red oak bows no less than 60", but I've also made a dozen board bows maybe, and more than a dozen others. Also, since I made the bow I can constantly monitor it for problems. A child will not do that.
Good luck, let us know how it goes and post pics.