To add to what Fuhnuh said, there are also lifejackets available at DC for anyone who wants or needs one. I know there are at least some near the dolphin pool because there was a little fellow (maybe 5 or 6) in the family who did the dolphin swim with us. DC staffers saw he was having some difficulty as the water deepened and offered to wait while the mom went up on shore and grabbed a lifejacket for him.
If you think your daughter might need floatation assistance in the lazy river, be sure she wears a wetsuit (they provide some additional buoyancy) and grab her a life jacket. There are sections of the lazy river which are way over my head...I want to say 8'. But no fear, there are TONS of lifeguards all along the way in the event that she finds herself in distress.
Enjoy your DC day! It really is lovely. If you want to see some recent pics, click on the August link in my signature and it will take you to my trip report. There's lots of photos of the lazy river area that might help!