Maybe consider changing the title of your post, because you are not planning to take them to the funeral.
I note that you say you would not bring them to the funeral itself, only to the "social part afterward." So they will not "get" what a funeral means. Either take them to both or to neither -- and with their ages, and the fact they weren't really involved with granddad at the very end and don't seem aware of what death really is, I would take them to neither. They won't understand that the social part (I take it you mean a reception somewhere with food?) is anything other than a party, and you could end up chasing them and telling them to behave etc. -- which takes away your and your husband's abilities to talk to the other adults who are there, to visit with relatives you don't see often, etc.
The kids will not get any sense of death or what a funeral is. Get a babysitter for both parts of the event and let yourself, and especially your husband, be free of the kids. You adults need to spend that time talking and sharing with relatives and his dad's friends rather than always having one eye on what the kids are doing. By the way -- what does your husband want here? This was his dad, but you don't mention how your husband feels about having his own children there. If he very strongly wants them there, take them, but if he's on the fence, mention that he might prefer to spend the funeral and reception really free to visit with the other adults.
Many kids as young as six do attend funerals, but you are not planning for yours to attend a funeral -- just a reception. I would not have them at either especially as they were not really part of their grandfather's final months; they were on play dates etc. That is fine! You and your husband needed them to be occupied during a difficult time! But they are not really clued into the process of death or grieving as they would be if they'd been around him much during his illness or if they were older children. Again, that's fine as a way to handle it.
One last thing -- you mention that you are "surprised/saddened that they don't seem to feel much sorry." Please ramp your expectations for them way, way back. They didn't see grandpa that often, frankly, and they are young children with no idea about death, and they were not there to witness him really declining fast. They should not be expected to feel and behave as adults or older kids would feel. They WILL come to you later and have questions, and be more puzzled than weepy or upset -- this is normal in children this age. My child had just turned six when my mother died and did not cry or seem to get outwardly upset but she was processing things, and did and said things later (sometimes much later) that told me she was thinking of her grandmother and processing the idea that she would not see her again. Please don't feel that your children should react in some specific way that's "appropriate."
By the way, we did not have the issue of taking our daughter to a funeral because there wasn't one for my mother, but if there had been I would have taken her; however, she had visited my mother in a nursing home several times, in the hospital, etc., and was aware how serious things were.