I am sorry for your loss. 
If you want to take them and you think they can manage it, then take them, she is your children's great grandma after all, they are allowed to say goodbye to her if you\they want to.
My MIL (so our DCs grandma) died last December, her funeral was in January. We had two teenagers and a 10 and 7 year old at the time and we took them all. We live a 7 hour drive away (plus time for stops, food, possible traffic problems\roadworks etc.) so had to travel on day one, have funeral on day two and travel back on day three, so quite a commitment.
They wanted to go to the funeral and it gave them the opportunity to say goodbye how they wanted to. Their 6 year old cousin wasn't going to go, but once BIL and SIL knew we were taking our DCs they let him go and the cousins all sat together with me and DH. I would have probably regretted it if we'd said no to them going as it was their only chance, we wouldn't be repeating the service for them later.
We did go ready to take them out if they needed to leave. Because DH was taking the funeral (he's a church minister) I would have had to of taken a child out if needed, but if I'd gone out with one then other family, SIL etc. were there for the others. We didn't need to take anyone out, but if you take your children to the funeral it would be a good idea to have a plan of action if you need to take a child out.