The 70/80's were the phenergan years. My mother admits to giving it us occasionally when we were being babysat. I have heard it leaves a nasty "hangover" in the children though.
I was prescribed it a couple of years ago when I had chicken pox, and I still carry 2 when I do longhaul flights. Its a last ditch effort to catch some sleep.

I have a friend with a autistic son, and they have to dose up their child on occasion when he is completely hyper.

Its not used as much anymore....... Probably not a good look to ask for it.... Times have changed a little.

And if my mum coped with 4 under 7 flying back from USA by herself in the early 80's with me throwing up and delirious, and there still being a Honalulu stopover without any medication, I think you and your husband can do the flight you need to without relying on the phenergan for such a little tot. Just my 10c.