If you want pictures instead of signatures you could do that, I think with this design it would be easy to add more squares. Also you could do a photo of each of your parent's children with their spouses (for those married) and then a photo of each set of grand kids by family, like suggested already..
If you do a wall hanging I'd suggest using the colors they used for their wedding, and the 70th anniversary is platinum so maybe something suggesting that too.