:thumbsup: to what everyone else has said, just a few thoughts on what has worked for me & friends (these are for if you bring the children into the service with you, rather than leave them in a group, which can also work, it's personal preference really):
1)It will feel counter-intuitive, but the toddler especially will be more likely to behave if you sit near the front as then s/he will be able to see & hear what's going on. Sit near the end of a pew so you can still get out if need be It also helps to give a quiet running commentary, ask them to spot various things, help them to hold the service sheet and sing the hymns etc.
2)Check where toilets, baby changing facilities etc are before the service starts!
3)This is up to you, of course (as it all of it), but I would advise avoiding the cry room if at all possible. I think they're a good idea in principle, but they're filled with fun toys, other children to play with etc and it won't take your toddler long to work out that all s/he has to do is make a fuss and s/he'll get taken away from the 'boring' service and into somewhere much more fun! This is fine if you don't mind sitting in there every Sunday for the next few years, but if you're hoping to use it as a one-off and then re-integrate them into the main service then remember how sneaky toddlers can be! You will have to take DC out at some point (it's inevitable), but I find it better to take them somewhere quiet where they can calm down without being over-stimulated; save the toys for when they're behaving well in their pew. If they need distracting when out, you can always point out architectural features around you (depends on the type of church but e.g. stained glass, decorative fonts etc). This is only one POV, obviously, I know many other people find it better to use the cry room & toys. Do what feels right for you.
4)Bring snacks that don't crumble everywhere/make a loud noise when you rummage around in them!
5)Most churchgoers won't mind if the toddler wanders up and down a bit. Set boundaries so they're not running around on the altar, but so long as you're happy with it, a little mini-exploration of the area around where you're sitting is fine.