Hi
I would suggest clicker training/positive reinforcement for a situation such as this. You can Google "clicker training" of watch YouTube vids of kikopup and zakgeorge21 to see a few training videos.
Basically, not all dogs
like children. However, aggressiveness should be strictly enforced as a no-no, like you said. Jackson loves the kids that he knows and sees often and will jump all over them, all them to put their face up to his, put hands in mouth and food bowl (we don't purposely allow this, btw, but it's good he doesn't care). He also loves young babies. However, I wouldn't say he "likes" kids. He gets pretty nervous around children he doesn't know (especially the hyper ones).
I started taking him to my little cousins little league baseball games (with children ranging from all ages wandering around) and many kids want to come pet the dog, of course. I'm totally fine with it and always carry a pocketful of treats. If kids want to pet, I nicely ask if they wanna give him a treat. So now Jackson associates kids with treats and the first thing he does when meeting a new kid is usually go up and sniff their hand to see if they have any, LOL. He's never shown his teeth, or growled, or anything, he just gets nervous.
Start slow... take him to a park or a playground but stay semi far away but where he can still them. Give him a treat whenever he looks at the kids to let him know... kids=treats. You can slowly start to get closer. Just keep doing this, it takes time. YT username 'Quicksilver' has been conditioning her dog to like skateboards so you may wanna ask her for tips on conditioning.
If he growls or tries to bite anyone while he's on your lap, take him OFF of your lap immediately. Do not pet, stroke, or 'reward' him in any way for this behavior. I notice so many people (including my father) when their dog is growling while on their lap and clearly stressed, they will pet her and try to 'relax' her when in reality, they are reinforcing the behavior. Maybe even keep him on a leash and the minute he growls or bites it's a firm 'ah-ah' and take him out of the room, then bring him back when he's good again. This method is shown quite a bit on 'It's Me or the Dog' on Animal Planet. I don't always think this method is the most useful but in your case, I do think it would come in handy.
Also, obedience training never hurts! Basic commands (sit, stay, down, come) and even fun tricks can boost a dogs confidence and can make them more well behaved dogs. Jackson loves doing tricks and this is a great way for him to feel more comfortable around kids. He loves to show off his tricks and the kids loves to watch. I've even let children do the commands to him and he will listen to them (so long as they have a treat in their hand!) so it really lets him see children as a leader and a fun thing!
Good luck, hope this helps a bit