What should I take ?

Maybe pamper them whilst you are there by ordering in a takeaway meal of their favourite foods, let them decide what they fancy at the time, and you pay for it on arrival, I’m sure that they would appreciate it.
A nice cake or some Luxury biscuits as a celebration, along with a bottle of Champagne, or speciality Teas and Coffees, plus some pretty flowers may be welcome too, but your most useful gift would be your offer of help with the day to day household chores, or running errands, a bit of shopping or even answering the door whilst the new Mum rests with her baby may be welcome, but best to ask first. Be careful regards buying toiletries as a gift however, l once did that as a wedding gift along with some very nice towels for a girl l worked with, and she was offended by it, saying ‘ are you trying to say we both smell?
Congratulations on becoming a Grandma, enjoy!

Yes definitely something for your DIL. I think the suggestion of bath products is a lovely idea. We all love to be pampered?

Definitely a gift for the parents esp. mam. My aunt bought me a lovely bunch of flowers rather than a gift for the baby because she said I was the one who'd done all the hard work and deserved a reward

Yes, DDiL really appreciated having a box of her favourite lavender scented Occitane bath products after DGD's birth.

My daughter is 43 , I received many gifts for baby but the only gift I remember from when she was born was from an uncle, it was a lovely set of bath products for me.

I agree, something for your DiL. You will spend the next umpteen years showering your grandchild with gifts, believe me! I remember when my last child was born, a neighbour I didn’t know particularly well brought a gift for her, and a box of gorgeous Roger and Gallet sandalwood soaps for me. She said people tend to forget all the hard work that goes into a birth once the baby is home! I was very touched. Many congratulations to you and your family.

Something nice for your DIL, would be a very nice gift.

Nothing else for the baby. Maybe a bottle of champagne or prosecco? And many congratulations. How lovely that they have invited you to stay.