If you look for a medium, be careful. Some will charge outrageous amounts with no results. I'm in Tampa Florida, and know several who will help you for free if you happen to be close by.
If its your son, he's attached to you, not the house. For you peace of mind you can remove the urn from the home, and do a cleansing. Removing the urn isn't necessary, but will allow you the comfort of knowing your intentions isn't towards your son.
It could just be residual energy, and not a spirit or ghost at all. If the home has a history of being filled with children's laughter, this is a possibility.
No matter what, if its only laughter, it appears harmless. How does the home feel? Is it light, comfortable, and pleasant? Have you tried simply explaining to it that it's now your home, and asking it to leave? You can always add that it's now the home of you, your son, and your daughter, that their scaring you, and they need to move on.