You can take them in to a seamstress/alterations place (or most drycleaners), and get them to "tack" them. Unlike Hemming, tacking just uses a couple of points to sew them. Then you just use fingernail scissors to cut the tack points when she gets taller. Tacking is more permanent than basting, but less permanent than hemming.
I DO know how to sew, but when my son had the same issue, we just didn't bother. Rolled the waist up and let him crawl around and undo it.
Here's the "in between" fix.
Elmer's glue & an iron. (We did this in the military to get "sharp" creases in our camis, and to attach rank -chevrons- when we didn't have time to sew them on.
Here's what you do:
- Take a clean pair of pants.
- Iron the crease you want (in this case, the length you want folded under).
- Drip a solid line of elmer's glue in that crease
- Iron the crease from the non-glue side
- Voila!
- In your case I would suggest doing a second line of glue around the "real" hem and wherever on the pants the hem meets (make sense? so there isn't a couple inches of "floppy" fabric inside the pant leg?)
Best part (or worst) about using Elmer's glue? It washes right out, and doesn't stain or damage fabric or your washing machine. So you have to redo any glue work you do, but it only takes a minute or two.