Our family has just combined and moved to a new town a month ago. We have three children, a 13 year old boy and 10 and 9 year old girls. The 9 year old is my daughter and the 13 and 10 year olds are my girlfriend's. The kids have been pretty quick to make friends in the new neighborhood.
Before we moved, we lived in separate apartment complexes and our children went to different schools and had different pools of friends. Our 9-year old was invited to many of her friends birthdays -- 2 or 3 each month. Our 10-year old was invited to 3 in the last year. Each time the younger was invited to a party, the older would feel crushed, but we would explain that they both have different groups of friends.
Today, one of their new friends delivered a birthday slumber party invitation which was addressed to only the 9-year old. This is a friend that both girls play equally with.
As a parents what should we do? We're not certain if it was simply an oversight, but that doesn't seem likely.