Lety -- With us, periactin started that way, too. He slept all day and was pretty crabby. It was a pain getting him onto the periactin, but we were able to counter the side effects this way:
What we did was start with a very small dose. So, originally he was prescribed 5mL once a day. That lasted for one dose. But, we had fought for this drug, wanted it to work, so we experimented a little bit. We first split the dose in half, so the goal was to get him 1/2 a dose in the morning and the other 1/2 dose at the end of the day. Then, we started with 1mL in the morning only. (Just one, very little dose once a day.) After 2-3 days of that, we went to 1mL twice a day. Then, I think we went up by 1/2 mL once a day, adding every few days, but we might have gone even slower than that (1/4 mL increase once a day every few days.) It took us a couple of months to work our way up to the full 2.5mL twice a day dose. (And, once he got to the twice a day dose even at the small end, after a day or two of extra sleepiness, he would eat more easily and gained a lot.) On periactin, there was one month alone he increased his weight by over 10% of his body. (Gained 2.5 pounds in less than 30 days, and we've kept it on.)
Worth a shot, maybe, during your off time from Megestrol or if it doesn't work for your daughter.
Good luck!
Erin