No matter what you decide to do you will have some people who believe you have made the wrong decision. Having said this you need to make the decision based on what you think!
Some people feel that to have children involved in a wedding adds a special magic. Others will feel that they can become an 'expensive' distraction. You may be surprised but many guests who have children would prefer not to bring them! A wedding is often one of the few occasions they get to enjoy a night out without the children.
1) Cost factor
2) Relationship factor
3) Make Invitations clear
4) Plan for any children invited
I'm sure you don't need to be told how expensive weddings are! Unfortunately most venues do not offer a discounted price for meals for children. Don't feel guilty when explaining to people that you need to limit your guest list numbers to control costs.
The decision to limit children at your wedding can be made in several ways. Some opt to include children of siblings only. This is quite clear cut and relatively easy to do. Most reasonable people will understand this decision. In my case we had the awkward decision of inviting some cousins and not others. We were paying for our own wedding and we decided to invite only my wife's 'close' cousins. We invited a handful of cousins who my wife had very close relationships with. As it turned out this offended one Aunt whose children were not invited. This Aunt declined our invitation. We were sorry about this but we were comfortable in the decision we made.
MAKE INVITATIONS CLEAR
Whatever your decision you should make that decision clear in your invitation. If you feel this is not possible you should visit or call those whose children are not invited to explain your reasoning. If you are excluding all children you can include this in the wedding invitations. One option is to have "2 seats have been reserved for you" shown on your invitation.
PLAN FOR ANY CHILDREN INVITED
The 2 ways people normally arrange seating at a wedding when children are invited is to-
(i) Seat all children with their parents on one or more tables, or
(ii) Seat all children together on a table
If you choose option 2 you would be wise to 'employ' a chaperone. There are professionals who are happy to attend a wedding and chaperone children- many of these will have games and ways to keep the children entertained. You may well know a teenager who would be happy to help you out by attending and looking after the children.
Once you've made your decision after considering all the factors move on. As we said earlier, you won't be able to please everyone. If you are comfortable with your decision that is all that matters.