Can I take snacks and water in?

Yes, they are decent about it.

There are picnic areas.. we take coolers for the whole day. Including sandwiches and snacks. Saves lots of money

Yes, outside food is permitted as long as everything is in a safe container (plastic, no glass or metal)

Yes

You can even get Subway or homemade food. Kennywood is one of the one amusement parks you can - it helps them if someone has special dietary concerns then they can bring something with them.

Yes as long as they are in safe containers... ie plastics