My kids are tummy sleepers too.

We have a breathing monitor that goes under the mattress and goes off if their breathing dips below a certain amount of breaths in a minute.

It goes off quite regularly with DD - we think she has mild sleep apnoea (sp?) and is a very shallow breather - but it would happen to her if she slept on her tummy or her back, so we aren't concerned its because she's a belly sleeper.

I just feel grateful there are things like the monitors out there.

Oh - and my DD has silent reflux too - so wasn't happy unless she was on her tummy so that made the deal for us!