Please help with 'party ' idea

My sis is having an ice cream social for my niece's 7 birthday..inviting 30 kids to meet for ice cream after school at a neighborhood ice cream shop
Sis doesnt want make big deal...too many presents (since it's only ice cream treat she doesnt want people to bring presents.) ...so it's to celebrate her birthday without going crazy. no mention of birthday on the invitation.but how could she incorporate birthday without asking for presents ? Short casual, ice cream treat
She is inviting 2 classrooms so that she doesnt leave anybody out since my niece has friends in both classes.

Ideas, comments, please.
What do you think of the idea ?