Lizzy - I too hope they take you up on the invitation, and I think you need to do a lot of thinking before then, if they do come. But please remember your invitation comes at quite short notice and they may have made other arrangements already - do not assume they are getting at you if they are unable to come - do not see it as a "test" for them.
When you see them, try and imagine those things that might make them want to back off - I have outlined some in my last post. Just make them welcome - do not try and hold the baby unless invited to do so; give no advice; express no grievance. Take every opportunity to bolster their egos as parents; criticise nothing; suggest nothing. Just enjoy with no strings. It can be a joy to watch ones own children developing as adults and honing up their parenting skills - they do not always do it our way, but I have learned much from my children as I watch them looking after their own.
If things improve and your GS dos eventually spend some time with you on his own, then reassure them that you will follow THEIR rules; ask for details of his routine and how they might want him treated in different situations. Do not be tempted to do it right (i.e. your way) for once while you have him.
It is only by accepting their rules and their parenting ways, and backing them up with encouragement and approval, that you will get this thing back on track.
The underlying criticism in some of your posts will not be lost on them, and is the last thing they need. If your son chooses to take this little lad around with him, and give him wide experiences (like eating in a motorway service station) then that is absolutely and unquestionably fine and it is not your place to criticise. You may not have done it that way, but that is their way - and he is their son, so that is fine.
