I started a thread that partly covered this a few months ago.
I completely agree that it's crazy to charge adults for bringing and supervising their kids at things that will hold no interest for them at all.
A lot of places offer a free 'carer' ticket with paid-for tickets for wheelchair users, which is great; however, there's every chance that the accompanying adult will enjoy the play/concert whatever just as much as the disabled patron. Not complaining at this at all - there aren't many concessions for what disabled folk have to contend with in life in general (or for full-time carers, for that matter); and there's also the very real possibility that a disabled person may want to go to the theatre, be unable to attend without assistance but not know anybody who shares their interest in the production.
However, if you're taking your toddler to Billy Badger's Bumper-Fun Bonanza, where you will get no enjoyment yourself (and as OP said, wouldn't actually be allowed on, even if you wanted to), you have to pay. It's not like they're going to go themselves, is it?
It's not exactly the same thing, but it would be a bit like 4 people getting a taxi to the station and expecting to split the fare 5 ways, as the taxi driver gets to go there as well.
I have no problem whatsoever with a ban on bringing your own food (as long as they don't only sell fruit-shoots, pom bears and turkey dinosaurs) - an adult buying a coffee will obviously directly benefit from it whereas a toddler wouldn't.