Cineworld are the best place for getting round this. It will sound like I work for them now (I don't), but they make cinema going affordable. Kids' club showings are £1 each or 90p if you join mycineworld (free) and book online. No booking fee.
Me and DS went to see Rise of the Guardians yesterday for £1.80, picked up tickets at the machine so no need to queue behind everyone buying the overpriced snacks.
Bag searching? Has never happened to me. I'm sure it varies from one place to another but like many mums I usually carry a largish handbag anyway and stash multipacks of Haribo, chocolate buttons etc bought at the supermarket in there, plus bottles of drink. It's always been fine. The staff are very nice (though as someone said above, if management are reading this, then they are very strict about searching
)
The other really good thing they do is that they're now doing the £1 kids' club showings all through half term. So ROTG is on every morning this week at that price, as is Madagascar 3, I think.
Vue is ok at kids' club (theirs is 1.50 but still with booking fee if you buy online, so I usually turn up and take my chances). Odeon version is two tickets for £2.50, I think (both child and adult) so not terrible if there's an even number of you, but less good if you're, say, 1 parent and 2 kids.
The other tricks to use are going on Wednesday if you can do Orange Wednesdays and get 2 for 1, or, again at Cineworld, going on Tuesdays when the prices are lower, again especially if you get the mycineworld discount. If I am going on my own, I try to go on Tuesday; if going with someone else, I go on Wednesday and get 2 for 1. DS gets regular kids' club visits with the occasional trip at full price but using any other discounts I can gather up.
The one downside is that my local Cineworld is pretty shabby, but I can live with that. The local Odeon is freshly done out and has whizzy massage seats and whatnot but then you are back to high prices, so I forsake it.