A lot of times you will find that activity increases in a location with the presence of a new baby or child in the household. There's a theory that their emotional energy attracts spirits. I also believe that it is also a matter of loved ones coming back to watch over or visit with them, or the spirit of the household becomes protective of them--even though they are unrelated.
Here's a story for you. I had a great-aunt who passed away in the home that she shared with her son and his family, which included her great-grandchildren--both under 9 at the time. My cousin told me about how the younger one of the great-granddaughters would be having conversations with noone present and complained one day, "Tap! Tap! Tap!
Pay attention to the details of their descriptions of them and things being told to them. Are they looking up or relatively even with their own height? It could be the difference between an adult and a child. What is the child saying? Does activity seem to center around or on the child? These are things to look for. Sometimes those imaginary friends aren't so imaginary at all.