Insomnia is a very common problem which we see often in the clinic. Most of the time it is caused by psychological problems caused by stress and anxiety. In general, kids have little problems falling asleep since there is relatively little stress in their lives. They don't have to worry about bosses, spouses, how to pay the bills and what is happening with their careers.
For young children, their world revolves primarily around their parents. I don't know whether you have come across the situation where the child gets upset when he see his parents quarelling. The child's survival instinct tells him that his parents are the source for everything, from food to shelter to love and affection. So, built into each child is a "Parental Bliss" alarm system which sounds off whenever the child feels that something is not right with Daddy and Mommy.
So it is vitally important for the child to know that everything is well between Daddy and Mommy and impetus is really up to us as parents to "Market" that to the child. Don't just take for granted that the child knows that you and your spouse are still in a loving relationship. You really do need to demonstrate it openly to the child. One of the things that you can do is to set aside some time each week for you and your spouse to be seen just talking to each other. This special "Togetherness Time" is a time when you and your spouse sit down and chat for maybe 10 or 15 mins and has to be a place where your child can see you but not interrupt. When the child sees that his parents are in a loving relationship, he will feel that his whole world is secure and life is good. That added sense of security might just solve that sleep problem!
My wife and I have been attending an excellent parenting course at a local church and this is just one of the many things that we learnt. So we have made a concerted effort to implement some of the things at home. It does make a difference for your child and for your family life as a whole.
If you have problems with your kids, maybe it is not your kids that need training, but you! The course I attend is conducted by Growing Kids Int'l and you can find more information at their webiste here.