Hello Peter,firstly welcome to you and your grandson the the forum. Your enquiry is a common one on here but my answers are my own and other people may have some other ideas! Your first port of call is to look through this site for a cornucopia of advice on fossil collecting in general - especially the pages about sites in North Wales - https://ukfossils.co.uk/category/wales-north/
. However, I, would advocate being a little more advenurous and seek out new / unreasearched sites by exploration. Start by researching an area online using terms like "geology + anytown" or " fossils + anytowm. There are also lots of pages on FB which are good. I will PM you.
First thank you for the warm welcome to the forum.
I did do I little research before posting and found reference to known sites at Welshpool and Llandudno, but was thinking along the same lines as you and fancied exploring somewhere fresh so to speak. However, most of the potential areas I know and which have public access, are on slate, hence my questions.
My general plan is to visit a couple or three areas first and then take the grandson once I have confirmed we have a chance of finding something.
Looking at his school planner, he has a couple of "show and tell" sessions in the New Year so would love for him to find something for one of those...