Kirsty i feel you pain. Alex has been doing them for months now and I've lost count how many he can do in one day.
I wish i had the answer in what to do but i don't. Gemma will grow out of it when and how long it will continue for i don't know.
I have in the passed joined in with Alex and his face is a picture he wonders what is going on and stops doing it.

Kimberley will have the odd screaming fit at the shops if she can't get her own way but i just take her away from that situation and she calms down. I'm waiting for the day Alex does it full on at the shops but at the moment he still sits in the pram.