My 7yo son loves baseball and the Yankees.
He wanted to play a real baseball game at his party. We decided to have a Yankees vs the Red Sox game and party. We made party invitations that looked like a game ticket. It had the two team logos and game date and time. On front the tickets said The Game Is On!" On the back we put the location of the party and asked everyone to bring their families and baseball gear so we'd have enough players for two full teams. We mowed out baselines in the field at church and put out bases. I made stickers with the 2 team logos for each player to wear and we divided into teams (the girls all wanted to be Red Sox and the boys were all on the Yankees team). We had 15 kids and 6 parents and grandparents playing. We also made banners to hang on the fences with chairs in front that read Yankees and Red Sox Dugouts. We played 6 innings and had a great time.
