What's the next best drug when keppra (levetiracetam) fails to control pediatric epilepsy? Staring seizures with full body involvement, now in clusters.

Not a simple quest. You should discuss with your child's pediatric neurologist. Depending on where in the brain the seizures arise from, type of seizures, frequency, and other medications and side effects acceptable to you and the neurologist , medications are chosen. Some medications work better on certain types of seizures. Not enough information here about your child and the specifics to give an appropriate answer.
It depends. There's no 1-size-fits-all seizure med any more than all kids eat 1 food or wear identical clothes. There are 3 basic types of epilepsy: 1) primary generalized, 2) partial-onset, and 3) everything else. "staring spells" etc is not good enough. That's not a diagnosis. Find that out. The "best" med is the 1 that reduces seizures with minimum side effects in your child. Arm yourself with knowledge.
Keppra (levetiracetam) dose. Another thing to keep in mind is that when doctors start a medicine to treat seizures in kids, we start at the lowest possible therapeutic dose for their weight/age. Then we slowly increase the dose if the patient has more seizures. Some seizures are difficult to control. But a second medicine should not be started until the first one's dose has been maximized. Your dose may not be maxed yet.