L.R.
Go for 11-1 or even 11:30-1:00.
I would issue invitations that say the party is for the guest kids only. Sorry, I know a lot of folks on here stress about inviting younger or older sibs to birthday parties and that only make things tougher. I bet parents would LOVE to drop off their five-year-olds and not have to stay, and not have to chase their younger kids while at the party. No one will be offended.