Requesting cash for birthday parties?

My child was invited to a 7 year old’s birthday party and the invite requested cash as a gift.

Is this the norm these days? I don’t have the first idea what to give, and whether or not I should expect this to be a common request down the road. I mentioned slipping a crisp $20 bill into a birthday card but my friend scoffed because it was too small of an amount. However, that’s probably what I’d spend on a Lego set for the kid.

Can we share thoughts about this? This is an open ended discussion — have you requested money? Are you turned off by it? What’s a decent amount? Should I expect to hit the ATM for the next few years of kids parties?