Your grandparents invited your family unit even though they aren't related to, or probably close to, your DH.
In our families, if a family unit was on some days 5 people and some days 3 people, then even if the original plan was for 3 people to come, if all 5 could come they would be welcome and we'd budge up, borrow chairs, put on a bit more food (although that probably wouldn't be necessary as all the hosts tend to over cater and sent food away with everyone). It just wouldn't be an issue, of course the children would come, it would be great to get a chance to meet them. We'd prefer that to half a family staying home.
I think it's interesting how evenly split the thread is, clearly other families have much more formalised meals that are precisely catered and inflexible, where extra mouths would be a burden and either rejected or accepted but resented.
What I'm struggling with is the idea that it is an imposition to even ask a day in advance if they could be accommodated. Because if the host is like me/my family where it wouldn't be a bother and a stress, there's no opportunity for us to tell the OP that (and she wouldn't believe us and think we were just being polite if we did).
Obviously she and her mother know the Grandmother and her capacity and the kind of events she hosts better than we do so she's the expert on this situation.
But in general in our families, expressing a "I hope I have enough food" before a family event is a bit of flapping from someone who has already catered for well over what we need to (as evidenced by copious leftovers), and not a deep distress.
I can accept that the OP knows her family better whilst still thinking that it all sounds very stiff, formal and not how I'd want family members to treat an event I was hosting.
There's a difference in my mind between the SC not being vital to an event with distant steprelatives (so being left out by it occuring whilst they are with their mum) and it actively saying we can't have them because of the event or they'll have to stay at home with their dad because they aren't invited/expected.
Some family events aren't much fun for kids, that's true. But if that's the case I think it's a bit weird to take one sibling and not the others, if Dad is staying home with brothers/sisters then I'd give the younger one this option too. Rather than reinforcing a blood relatives versus step children division.
I know you didn't ask for this this weekend and it made sense to you that your unit of 3 do stuff with your relations when you are 3 and not 5. But maybe you need to think about whether that habit means that you can't turn up at your relations as a unit of 5 with a day's notice when that's unexpectedly an option.
Out of genuine interest, because I struggle with these unspoken social rules if the SC would have been welcome if it was their weekend, and one day's notice was too short, how much notice would have been acceptable?