We haven't used this particular drug, but our son Jon has significant issues with aggression and agitation with other medications (and even the seizures themselves have an impact).
We just started him (actually yesterday) on a magnesium supplement, and today, he was SO much better -- not aggressive at all, very calm, smiling...
I know it's just one day, and will have to see if the effect continues, but anyway...magnesium also may help with the seizures themselves (and also with sleep).
As Nakamova recommended, I would persue the diet with your neurologist. Many doctors wait until a child has gone through numerous medications before even mentioning it, but we found it very helpful -- our son was able to be seizure free for 11 months, and part of that time on no meds at all. He recently relapsed, and is back on medication, but we are also trying out different nutritional approaches to controlling the seizures. The less meds the better.
Our nutritionist has said that a lot of kids can be helped by some simple modifications to thier diet, without having to go on the strict ketogenic diet. She suggests removing sugar and gluten (wheat products) from the child's diet to see if this makes a difference. For some kids, this will pretty much bring about seizure control. We gave this a try for a couple months before Jon started the Ketogenic diet, and it did reduce his seizures a lot. However, we didn't realize that stuff like ketchup and BBQ sauce has loads of sugar in it -- so it pays to read labels.