I believe the rules say none at all, but I brought a stroller in several times with a couple bottles of water and small snacks for my little (4 and 5) ones, clearly visible, and they didn't seem to care. A couple well timed gummi snacks or peanuts can go a long way in keeping the kids happy on busy, hot summer days.
As long as you're respectful about it, I don't believe KI really minds...