We've been in cool weather many times and the kids do fine. Just make sure they have comparable insulation to what you will take for yourself. I like to hang side by side so I can be close (and warm) if they need something. If during the night, I can tell it's gotten colder or windy, I can just reach over and make sure they are still tucked in and toasty.
The link mentioned in an earlier post has lots of good info.
My 3 girls set their all time low (24*) at the FL hang this past January. They had 20* down UQs, Kelty 30* synthetic mummy bags, and slept in their Capilene long johns. I also make them sleep with their jacket/fleece with them inside the bag. It'll be warm to put on in the morning.
Get some of the handwarmer packets and teach him how to open them. Let him sleep with a couple in his hammock. If he does get cool during the night, he can pop open a warmer on his own.
If your plan is to stay multiple nights, have a bailout plan if things go south.
Hope you all have a great weekend. Camping with your kids is great fun.