My church is large, and no one "official" would notice if you were there or not. However, often the same folks will attend the same mass week after week, become "regulars" so to speak, and if someone is missing, onne of the other "regulars" might call to check, more out of concern than actual checking up for a head count.
If your child is in the parochial school, I believe they request you use envelopes as a system of tracking Mass attendance from the perspective of supporting and practicing the catholic religion. I don't believe the amount of $$$ you put into the envelope has anything to do with anything. It's more like they want to know that the parents of the kids are fostering good religious practices.
I don't have children, but I have heard from folks in my parish who do that the parish is quite flexible in arranging payment plans etc., so that all children who want to attend can. It is not based on financial contributions, but more on participation, time contributions etc.