We went to it a couple of years ago, the first time DD was a bit older, maybe 6, the second time maybe 8-9.
It is not a church service, it is a carol concert. The conductor was great, introducing instruments and how the carols tell a story and how the instrument work together.
It is more a classical one, not pop song or what is played in the radio so it depends a bit of what your child is used to hear.
Not sure if it is on again or if, it may be sold out, but we went with a group of friends to Cadogan Hall a year before and that was definitely aimed more for younger children. Their family concerts are great in general but tickets are snapped up very fast.
These concerts can be great to introduce children to the concept of a bigger concert and a lot are absolutely child friendly. When DD was 7 we ended up in one at the Royal Albert Hall, they happily encouraged children to sing along and nobody said anything, most adults were in a happy mood.
We try a proper grown up classical one this year, DD is 12. Will be interesting.
We are not religious so apart from the school Christmas service we don’t go to any church service but a genuine carol concert is very different. And St. Martin in the Field is a beautiful venue.