I second the vote for Looney Tunes, and also Tom & Jerry. It isn't politically correct to let kids watch such terrible, violent cartoons now (stupid bloody view), but they are fantastic. And when my cousins kids watched them, they were literally hypnotised and silent for a couple of hours. When it was time to go, they turned deaf and refused to budge!
You can't go wrong with Disney classics (I don't mean Aladdin or Lion King!). 101 Dalmations (even the live action version), the Aristocats, Jungle Book, etc.
Personally I think one of the very best kids films ever is Mary Poppins. In recent years, I have a soft spot for Matilda.