If it were me, I think I'd at least consult an attorney on this one. She might be able to get the penalty lowered if she appears in court, but I'd want to talk to someone first.
You said:
She was told she doesn't have to attend court, she can call after the the court date to find out what the fine is.
If the officer told her that, it probably means they aren't seeking jail time, but want money. If she has her own auto insurance company, she might want to talk with them (annonymously or not) to see what those repurcussions will be too.
She at least needs to find out what she can expect to happen. She's got a lot of responsibility requiring advance planning. No one likes this kind of surprise.
I got a speeding ticket in Virginia once (Caroline County) about 6 years ago, and ever since then I am super conscious of what speed I am going even if someone else is driving... especially while in Caroline County. They will never catch me speeding again! That ticket was for well over a hundred dollars and I got points on my record in Maryland which stayed there for 3 years, but my car insurance company had no reason to find out about it. They only would discover it if I subsequently had an accident and my record was checked.
I suppose getting a speeding ticket is not such a bad thing if it causes one to slow down and be more careful.
Keeping kids belted is whole another thing. Maybe that particular child needs to be in court with Mom to see that she's in serious trouble for it to sink in that the belt is to be worn. Some kids don't feel safe in a moving vehicle if they're not belted in. They wear a belt in an amusement park ride and they have worn a belt in the car since the day they came home from the hospital. Other kids don't seem to be imprinted like that. Not sure what the cure for that is... other than supervision.
Is it possible the child unbelted when the police stopped the car and popped up to look around? Was the child in the back seat? Technically, if the car was stopped when the kid was unbelted, the belt was not required...right?