Sazz, you seem to be somewhat disappointed in your grandparenting experience, and I'm sorry about that. I'm not clear, though, on whether you and DH are the only adult relatives excluded from birthday parties, etc., or if the parents aren't inviting any GPs or other family members. After all, as PPs have said, it's possible that tickets to some events are limited. It's also possible that they don't want adult relatives at parties for their kids and their friends. Rest assured, it may have nothing to do w/ you and DH.
"Son brings the children to see us every 2 or 3 weeks sometimes DIL comes with them."
IMO, "every 2 or 3 weeks" is good for GPs today. Young families are very busy, what w/ everybody's different schedules. Many GPs see their AC and GC much less often than that. DIL may avoid the visits b/c she wants to leave you time alone w/ DS and the GC or she might enjoy having the time to herself. Or yes, she may be responding on the basis of some underlying tensions between you. IDK, of course. But regardless, IMO, it's still good that you see DS and GC on a fairly regular basis.
" I usually learn if they are ill, going for a day trip, or in the Nativity Play from Facebook which is not really very good."
Maybe not, but it also seems to be very common. Iv heard of/read about it before. And I often learn things about my DD and kids via FB. For many young people, today, that it simply THE way to communicate. They probably feel as if they are "telling everyone" when they post something like that on FB. And they most likely assume you've been informed by them that way, too.