"This is not difficult. A tiara for the girl. A cape for the boy. Done. 5 Below or Party City will have very cheap options. "

+1

My children are close in age so in preschool, they were often invited to the same birthday parties. For one the "princess" birthday parties, one child loves princess stuff and dressed like a princess. My other DD went in shorts and a t-shirt. No one seemed to care. The Dollar stores and the bins at Target with $1 and $3 items have some princess stuff that you can buy. It's not a big deal as long as the children enjoy the party, who cares what they wear.

My sons would have had a blast at a princess party at 5. They might have worn "prince" attire, but they would have been perfectly happy with the sparkly fairy wand or tiara from the party. They loved to dress up.