Next time you see you son and his girl friend, do the following.
Tell them that you have promised yourself not to be the kind of grandmother who continually offers advice, she has not been asked for, so if or when they feel the need of advice after becoming parents, please will they ask for it?
You are saying this now, because when you were a young mother your mum, MIL, assorted aunts and other female relations all felt free to give conflicting advice and critzise whatever you did. AND IT WAS PURE HELL FOR YOU.
You will be happy to help, but again, when you are asked.
AND stick to this, though there is no harm in ringing to hear how they are getting on at reasonable intervals.
Immature or not, they are physically old enough to have children, neither suffer from diminished responsibility, so stop worrying.
Most likely, they will run into the snags all new parents do and cope with them.
If they should not be able to cope, then you will need to find a way of offering or insisting that they let you help, but don't waste time and emotion on worrying about something that may never happen.
Most young parents learn a heck of a lot from their first child and they and the child thrive on it. And the grown ups turn into responsible grown ups in the process.