I found this mini rainbow balloon arch listed as a St. Patrick’s Day party idea, but I think it could just as easily work for a child’s rainbow themed birthday bash. Or, if you wanted, you could simply adapt their instructions to make a plain arch using balloons that match your color scheme. Visit Oh Happy Day for the complete tutorial.
Hosting a Rainbow party? Check out these Rainbow party articles over on our Party Ideas Blog and these Rainbow party decorations we found on Amazon.