M.S.
I have 3 young kids and seem to have neighbor kids over a lot. We have a large bonus room as a play room and the kids have free reign over lots of the house. It is 2000 sq ft so not huge and I am within earshot. So far, they all play really well together, no issues and mine share so well because they just don't know any different. So, in my opinion, as long as you have witnessed the kids playing well together with no hitting, biting, etc, then as long as within earshot, let them have fun! :)