I hate to be the negative one in all this... but we did this at our wedding - which was not interstate. We didnt invite children, and we had friends that RSVPed with their child's name. We told them that we would prefer they didnt bring the baby.
This had nothing to do with cost... it was simply because we hadnt invited kids that were a lot closer to us than them and their child, and we didnt want others to get offended when they saw their child there. They told us they werent comfortable leaving their 5 month old with a babysitter so they came to the church only and not the reception.

P.s. A few weekends earlier we found out they went to the AFL grand final together and left the baby with their grandmother...