as red suggests, may be they should speak out loud and ask for the spirits to stop. Most poltegiests are harmless and just want you to be aknowledged that they are there, if they are children then im sure they just feel like there playing. Try and find a local medium/clairvoyant in your area and ask them to do a home visit perhaps, maybe they can find out what happened to them, if there was a fire etc. Another suggestion i can give is to give the nearest university a call, there are paranormal phycology courses all around the place and the students would love to use this as a case study.