My husband's parents looked after our baby when she was sick because we had to go to work. They took time out and looked after her overnight. Should I get them a thank you gift or is this just something grandparents do?

I would get them a thank you gift, or at the least write a thank you note. That was very kind of them. While most grandparents who are able to do that are glad to do it, it is still taking time from them and the had to rearrange their schedules. It also shows that you are not wanting to take advantage of them.

That's just what grandparents do and they probably loved every minute of it. You might want to get them a cheesecake, or something similar, and have them over for coffee as a thank you.

I wouldnt say a gift is neccessary .. But, a big thank you, and maybe an invite for dinner to spend time together would suffice.

You can rarely go wrong with a thank you note.  A gift might be a bit much, because they are family, but a homemade treat, would be thoughtful.

I have to agree, with most above, I'm sure they don't think it's necessary to even thank them "officially", but a thank you card would be a welcome gift and a show that they are appreciated.