You should try out Lu.ma - it does all those things for free. I use it extensively for managing rsvps to both private and public events.
- ability to set each invitee as an adult, a child or an infant
- ability to group invitees into families, so that any adult in the family can edit the RSVP for any/all family members- adults don't need nerf guns
- kids and adults have different drink preferences.
Ability to group invitees into families:
- whichever parent is 'in charge' of plans for that weekend can respond
- whichever parent realises there's a change of plans can edit one or more of the RSVPs
- often a family will accept/decline differently for different members, e.g. older sibling isn't interested, or one parent will run errands whilst the other is at the party