I've definitely noticed a trend in children who do not know how to play by themselves (especially non-school aged children). I, personally, blame it on portable video game consoles. It seems that younger and younger children are being given these gaming systems to play with. I once had an interview with a family who had 3 children...a 5yr old, a 3yr old & a 1.5yr old. The 5yr old did not interact with me but played with his portable video game console the whole time he was here. The 3yr old seemed lost among all of my son's (who was 2.5yrs old at the time & had, in my opinon, some pretty kick-butt toys) toys and kept asking me if I had a Wii or another type of gaming system (a big no). Only the 1.5yr old seemed interested in anything (she was busy exploring). Needless to say it was a big no to this family.
I agree with Crayola Kiddies. Leave them and eventually they will learn to play on their own & with each other.