Are you asking whether Year 3 & 4 players can take part? I would say Yes, the more the merrier.
Tag Rugby is a Non-Contact Sport. So Age and size difference should not matter. Though generally speaking Year 6 players can be significantly faster than Year 3 pupils (sorry to generalise)/
Having said that, last year, the winning team - Sevenoaks Primary - had Year 6 and Year 5 Boys. The girls in the team however, were all Year 4.
Hope that covers it?