I would be honest personally. Keep them open-minded, but not fill heads with crap.

Santa Claus is a different kettle of fish entirely. Its an act, we all know its an act, its fun and if a kid starts to get too old without figuring it out then its not unusual for a parent to tell them the truth ultimately to avoid embarrassment.

God OTOH adults believe in. Its not something you grow out with so to fill their heads with one thing now, then something else later is going to be confusing.

My personal approach (never having needed to do it) would be to explain as well as possible why it rains (depending upon the age of the kids) and that different people have different views on god but that you don't personally believe it exists.

Just my own random thoughts, I think a lot would have to depend upon the age and maturity of the kids. I would try to avoid it rains "because god says so" preferably though.