It might not actually be a children's mass at the other church. I have to admit, I don't know much about the United Reformed Church, but the catholic church we go to is really friendly towards kids. On Sundays there's a children's liturgy, where they can go over to an attached room and do fun activities like colouring (the school-age ones, anyway). If you have a young child being really noisy, there's an area you can stand in that has speakers, so you can still hear what the priest is saying, but the baby noise isn't stopping other people from being able to hear. The mass is a normal one though, not an actual children's mass.