You've tried talking to the dad, obviously that doesn't work.
If the children are in your yard without your permission or adult supervision, I would tell them to leave as you did before. If that doesn't work call your local police department. At the very least the 9 year old isn't old enough to be responsible for himself, and at worst they are trespassing.
We actually called the police once because of how loud the neighbor kids were. Normally we didn't mind their garage band playing, drums, amplifier and all but we were trying to get our baby down for a nap. First we politely asked the kids if they could keep it down for an hour. They turned the amplifier down for a while then said we had to talk to their mom.
The mom basically said she had to deal with everyone being loud when her kids were babies so suck it up. So we called police, and the nice man in uniform came out to measure the noise level and they were breaking the law based on the decibel level. We never had a problem after that (other than a few dirty looks aimed our way by the mom).