If we are just going adults, late teens and up, there is only about 75-80 Windsor/Spencer guests. Add to that Meghan's family, maybe 100 family. That leaves plenty of room for 200-300 friends, and still 200 or so people from their charities and work. I highly doubt they sent out invitations to people 'they know will not attend'.
I don't see them dumping family members off the guest list. It's not the same as having an intimate ceremony, and deciding not to invite your second cousin once removed because you want your college roommate and husband to attend. Some of the charity representatives Harry may have met them alone a handful of times. There is no need to invite their spouse to attend. Work invites are often just to the individual at a normal wedding. I don't see them not inviting the Duke of Kent, so some worker for center point's wife Harry has never met, can attend.
There will also be many people from their charities outside, among the 2000 invited into the castle grounds to watch.
Limiting it to 600 also means they can invite everyone to the dinner. Instead of saying 'oh well you can come to the ceremony, but not stay for the party'.
So, are we all thinking the reception after the wedding is a childless affair, excepting the wedding attendants, if they are children (and I think they are)? And that the evening event is adult?
The day time reception will be everyone from the ceremony. So if any kids are invited to the ceremony, they will be there. I wouldn't expect any, other then bridal party, and perhaps some teenagers.
The evening party will be late into the evening, so I would think just 200 adults. Not the kind of thing you invite kids to.