I think there are two issues here, one is how to gently reinforce positive behaviour in your cat without resorting to not nice words or smacking. The other issue is how to teach your cat and your chlid the "rules".

The first one is to parent your pet like you would your child. If your child were physically attacking someone else, you would remove them from the situation and let them calm down before letting them back in. or I should say, "I would...." So do the same with your child. If your cat is meowing non stop and being generally irritating, you can talk back and carry on a conversation... or you could ask them to be quiet... those are gentle options. I know I say to my girls often "Geez that dog is being annoying by barking, lets go see what is going on" so they see that we need to resolve it and not leave the animals to carry on.

I'm not against the use of water pistols for more dangerous behaviour, or for scratching furniture etc etc... cats hate them, but I don't think using them for meowing is something you want to show the kids. I mean its basically saying they get in trouble for talking? My girls have "quiet time" when things are too loud, and if they keep going they go to their room. So I would consider if the cat is too annoying to give the cat somewhere to go. Away from the situation.

The children vs animal thing... well that is a hard one. I often have had to tell the animal that no matter what it is not okay to scratch/bite/claw a child. It doesn't matter how it happened. I think its important to teach our children how to behave around animals and animals to behave around children so that the situation isn't repeated.