Sorry for your loss. I would prepare them for the fact that they will see adults upset. Also that they might get upset and it's fine (also fine if they don't). If they find it really hard, they might want to leave the service ... Is there an adult who could go out with them if that was the case?
I agree to let them know it is ok to be upset and ok not to be upset. Make sure there is someone there to comfort them that they are happy with. I remember being upset at a funeral and my dad busy talking to someone else, so a random friend of the family accosted me with a big suffocating hug that I didn't want.
If in doubt copy what everyone else is doing. Don't use your phone. Take tissues.
Maybe talk them through the funeral itself, whatever you have arranged. Gives them a chance to ask some questions and also not leave them wondering what will happen/is happening on the day. Definitely agree with Cheery re having someone lined up to be there for them who won't be too overpowering. If you are having some kind of a wake, is there something they can help with there e.g. handing round drinks/food just to keep them involved and busy, and so they feel helpful and doing something for their Gran.
Thanks for all your help and advice. The funeral went very well, quite light hearted and touching. It's funny the one I thought would be really upset wasn't and the one I thought would be ok cried all the way through. That's kids for you I suppose, keep you on your toes.