My children are adults too, but my granddaughter was here 3 days a week for 2 years before I decided to open a daycare. She was old enough to be excited to have friends joining us, but there were times when you could tell she wanted "just Grandma and me" time. I made sure to give her lots of attention when the other ones were napping. I just read her a story and we cuddle and talk about the fun we'll have when the babies are ready to join us again. I know your son is only 11 months old, but I would make sure to give him lots of extra attention. I expect once he feels more secure with the new arrangement, he'll enjoy having a playmate. It's sort of like getting a sibling... many children resort to infant behaviour and clinginess when a new baby joins the family.