Teenagers can choose when they see their parents, especially if they live close by, unless there are safeguarding concerns, like previous DA etc. They don’t have to be ferried around or ‘handed over’, unless they’re at risk in some way.
The most important thing for kids is their parents put their needs first, get on as well as possible and they don’t feel caught in the middle of a parental battle. It’s in their best interests to spend as much time as possible with both parents, so should be encouraged to do so. If kids feel their mum doesn’t want them spending time with their dad (or vice versa), they often choose not to, to avoid upsetting their mum. That’s not acceptable. Amounts to emotional blackmail.
But time with grandparents is also important, so if they like being with their grandma, that’s fine too.
Suggest you try to avoid court and ask the kids what they want to do when you’re not around, making it clear if they want to be with their dad, that’s great.
If they say they’d rather be with grandma, their dad should accept it. I believe a court would support the preferences of the teenagers.