Change in attitude

Do the parents mention it? I think it is quite normal for kids to 'let off steam' when they get home, but you don't say what sorts of behaviour makes it 'unbelievable.'
Also you say DH drops them off- but you aren't there to witness it yourself?
If he finds it stressful why doesn't he ask for parents to collect from your home?
Does he stay in the family home until the parents come back, is that what you are saying?
I think I would leave this to the parents and DH to sort out...

When I was a childminder exactly the same thing happened the minute the parent arrived to collect child(ren).

I always made sure they were ready to leave immediately and I left it to parent (usually mummy) to deal with behaviour

Does DH need to stay at drop off?

DH drops them off as it's always been the routine. Both parents normally there but sometimes just the one. Shouting, screaming, fling their belongings on the floor, sometimes they get thrown across the floor. It wouldn't matter who dropped them off this is just how they are and goodness knows when the calm down. I feel so down as they leave here in a good mood, had fun, and there is a point on the journey nearing their home they seem to change. Makes me feel we have done something wrong.

I don' t see how this behaviour can be a reflection on you.

I do however think you should leave it to the children's parent, or parents to deal with , as it occurs after they are back in their own home.

In your DH's place, I would go in with the children, say hullo to whichever parent is there, then say "I'll be off now." and go.

Icandoit, I’ve been there and felt it must be my fault but have learnt that it is very common and nothing I did would make any difference. It’s just the way it is so stop worrying and enjoy being with your grandchildren.

Kids don't have enough freedom nowadays to get any rowdiness off their chests without adults being present. Your grandsons' safest place for "letting rip" a bit is with their parents and in their own home. The fact that they know how to behave well the rest of the time is a credit to them and their parents.

They are probably tired/over-excited/glad to be home.

Yes, all credit to their parents and GPs that they are fine in someone else's house.

If it happens in the car can you go with him to drop them off?

Whether you go or not, I'd say don't go in!

I wouldn't worry. What is the alternative?

My gran used to say we were as good as gold until a parent arrived then the devil took over. She used to wave us off with both hands. Now we look after our DGD and she’s the same she fine with us and tells her dad to go home, creates then 2 minutes later is all smiles. I’m sure your grandchildren are just letting off steam.

This has nothing to do with you at all. The parents have to see what is happening, the children probably need to link up with their mum and dad. You look after them as best you can... if the parents feel this is not good enough, they have to look at their life style.

Advice/comments - noted. Thank you all.

My daughters used to come back from visits to Granny and Grandpa with exquisite manners!
DH and I would notice it when we went to collect them (usually involving lunch)
It may have been because my PIL were lovely courteous people, FIL an old fashioned gentleman in every way.
“Please may I have the salt/butter/gravy”, “Can I help set the table, Granny”, “Thank you for lunch, please may I get down?”
We used to think they were changelings!
We reckon it lasted till about half way home when they reverted to being our usual stroppy kids, but it was lovely while it lasted!

My two grandchildren are the same they sleep over twice a week, and as soon as mum or dad comes to pick them up they start playing up