I have put together a special sheet which I give out to child guests at christenings, explaining the various parts of the service to them. I find it goes down very well with children (and adults). The children can take it away with them, colour it in during the service or afterwards etc. I have uploaded my copy, but it is really something that needs customizing since every church will do baptisms in a slightly different way, and probably want to explain the significance of the various features in different ways, so this is really just an example of one way to do it.
I have found this leaflet a good way to engage with child guests when they arrive – they like getting something special that is just for them. (I also include them in the service by getting them to light the Paschal Candle and pour the water into the font.)
I have found this sheet is also appreciated by local schools who bring children to visit the church or ask me to go into school to talk about baptism.