OP,
If your son is so stressed, he could sit down with his wife and agree on the budget - with or without her coffees
I wouldn't a person like you taking care of my child so "I pull my weight and contribute"
You clearly do not like your dil. I bet you also do not approve of her parenting, for example.
Your son doesn't help with childcare his son - he is parenting his son.
I hate the word "help" when it comes to a male doing his share of the work.
I still say back off. Like 100 ft off. Let them fix it by themselves.
If your son complains to you, tell him to address it with his wife.
You are not part of their marriage. Step back. You have no voice in their marriage. Just another meddling mil who complains how her baby boy is being mistreated by his wife.