Depending on the type of reception you want, you have 4 options: 1) welcome them with open arms; 2) choose an adults-only wedding; 3) only include children from the immediate family; 4) hire a service to look after and entertain the children, usually in a separate room.
It’s not a rule, but you should expect 30% of the people you invite not to be present. This will always depend on the location of the wedding, the number of guests that are from afar and on the time of the year, since it might coincide with previously booked holidays.
Give your guests at least 15 days between receiving the invitation and the deadline for replying. Sending a sealed envelope with the invitation or allowing replies via email might help getting faster replies. A week before confirming the final guest numbers to the suppliers, call the people who are still in doubt (this is the time to ask the godmother or another friend/family member for help).