"I would like to take the kids to get whatever that want for DH.
Is that my place? "

Yup - That's it!
You get to pay for the gifts too ;-)

As to dinner arrangements, talk to your husband and see what he'd honestly want. He may or may not have a strong preference -- either for dinner alone with his kids, or with all of you.

it would be fun to have dinner with your dad, SKs and DH together, ask how your dad and DH feel about it and go from there. if DH would rather be with SKs, then you go see your dad.

it is Ok to take them shopping for dad.

SS8 likes to make dinner for Father's Day, so I help him to do so. I help him while he plans the menu, he comes to the store, scans the food through the self-checkout, and does as much of the food preparation and cooking as is safely possible at his age. It seems to mean a lot to both he and DH that he went to much effort to prepare a special dinner when he could have been playing. And it gives me a great opportunity to pass on grocery shopping/kitchen safety tips that he will find useful when he grows up.

The funniest part is that he really likes to "surprise" Dad; but he must think poor Dad is either feeble minded or hard of hearing or both. SS is always discussing dinner plans in a stage whisper from the back seat of the car while DH is sitting right next to me and assuming that DH doesn't hear! DH is, of course, always very surprised.

Sadly, this year he will not get to do so. The temporary court order was for every third weekend, without exception - so he will spend most of Father's Day at BM's. And with incredibly bad luck SS is also missing another birthday party, poor kid.