What do you call the action when you press your mouth against baby's tummy and blow air to make the baby laugh?
That is typically referred to as blowing raspberries:
Blowing a raspberry, strawberry or making a Bronx cheer is to make a noise signifying derision, real or feigned. It is made by placing the tongue between the lips and blowing to produce a sound similar to flatulence. In the terminology of phonetics, this sound can be described as an unvoiced linguolabial trill. It is never used in human language phonemically (e.g., to be used as a building block of words), but the sound is widely used across human cultures.
Here is a Google search as they relate to baby tummies.
Zerbert is what I have always heard it called. Several sites of mixed authority confirm.
Although there's evidence provided for "zerbert" and "blowing raspberries" providing the meaning you're looking for, it's worth mentioning that in many places, there isn't a particular word or expression for this except for blowing on a baby's belly.
If I (from California, US West Coast) went up to my sister and was like, "I'm gonna give you a zerbert," she'd probably think that I had a piece of candy or dish of sherbet (frozen non-dairy dessert, kind of like ice cream) to give her. On the other hand, if I said, "I'm going to blow raspberries," she'd then expect me to stick my tongue out at her and blow.
(of note that the urbandictionary link's #4 definition provides the etymology of zerbert, which would be of use to read.)
So, be advised, if you're going to use either of the above phrases – unless you qualify them with additional words like "on the baby's tummy" – you're just as likely to get blank stares or misunderstanding as you are to be understood. There's a big difference between "blowing raspberries" and "blowing raspberries on a baby's belly."