2 x Phernergan 30 mins prior to flight
Don't! or at least not without first speaking to your GP.
Urshtnme - that is a very dangerous recommendation.
Quite apart from the fact that you don't know anything of the little girl's medical history which may include a condition or therapy for which Phenergan is contraindicated, Phenergan itself comes in several strengths and ONE 10mgms tablet is enough to make me sleep and I'm an adult.
To everyone else, please be VERY wary of taking ANY medical preparations without first discussing it with your GP. The fact that something is recommended by anyone who posts here does NOT mean it is safe and many of those who make such recommendations have absolutely NO medical or pharmacological training. I am qualified as SRN, RSCN, (that's the qualification for sick children) and SCM but would never suggest that anyone takes anything other than substances KNOWN to be harmless, such as ginger or uses any other non-drug remedies such as the bands mentioned earlier.
Atishoo have you thought about taking her to a local airfield for a short trial flight? You give your location as London so why not go over to White Waltham? There is at least one FI there who is also an airline pilot so is likely to understand the problems involved and be sympathetic. Please feel free to pm me.
For those who think I may be promoting WW, I'm not, I have NO vested interest in the place, I'm simply aware of that particular Instructor's existence.