It is a scientific fact that grandparents have forgotten every damn thing they learned about child rearing, because they never did any of this crap when I was a kid.

Unsolicited Advice is mostly useless for two reasons;

Firstly context matters. Since your context is different to mine, my advice may or may not be useful. For starters children are different, so what worked for me may not work for you.

Advice -before- the first birth is even less useful. They have a plan. Your experience is meaningless in the face of the plan. When you hear the plan you don't need to comment on it. Smile, nod and agree. Don't worry, the plan won't last very long...

Mostly parents don't need advice. They need support. An ear to listen. A voice to agree this is hard. A prediction that it gets easier (it does). A view that they're doing a good job (even when it doesn't feel like it.)