Make them feel welcome.
On the day of your wedding you don’t want to focus your energies on arguing with a guest so just let it go and let the caterer figure out the seating issue. If you want to say something you can but unless you ask them both (or the family) to leave you’re probably going to get into an argument with the end result that they’re staying. Why not enjoy your wedding and say something after you return from your honeymoon.
As for costs, the caterer will probably tack on an extra fee for the adult guest but kids will either eat free if they are under 3 or pay a reduced rate.
This situation is also ideal for your wedding planner (day-of or full service) to handle or designate a member of the bridal party or close relative/friend handle any situation that arises.
Remember this is your wedding day. Don’t let anyone ruin it for you.
And if it’s an RSVP card that comes back with uninvited guests listed as attending you can call the invited guest and tell him or her that you’re sorry but they can’t bring a date or their kids because the space doesn’t allow for it.