my mum taught me a trick, fill the house with white light in your head, so that it shines really brightly from the inside out, then NOTHING that means you harm can enter. if the young boy is still there then he means you no harm and is nice. as was sadi above, he could be a protector. find the room he most visits, and sit quietly and ask him to tellyou why hes there, and if you want politely ask him to leave, tell him that our family is there now and its your house. also sorry you have kids? i forgot to look??
make it plain that there is to be no funny business whilst the kids are sleepign only daytime visits. kids are usually very sensitive to people.

good luck.