Get the child support agreement in a written contract, and keep receipts for EVERYTHING
Buddy of mine separated from his wife, but she kept the kid and he paid her child support, but nothing that was written up formally or decreed by a court or anything. Year or so later she got pissed at him for whatever reason and went to court claiming he had never paid her any child support and she owed him a ton of back pay. He didn't have much proof that he had been paying here, he often just gave her cash, so he basically ended up having to pay again.
Things may seem amicable and agreeable now, but you should still do everything you can to protect yourself, there is no way to predict what will happen years down the road, people do change.