Consider how you will use the RV most and how often the grandkids will be joining you. For example, are you going to drive to the lake for the weekend, park it, and let the kids play outside? Or, are you taking the kids for the summer and crossing the country and seeing all of the national parks? (All will be inside for much of the travels.) Will the grandkids be with you only once in a while, or do you provide child support while the parents work? The answers to questions like these will help you focus on the kind of RV that works best for you.
We have a 27 foot RV, and our two adult daughters and sometimes our Son in Law travel with us. Often we take long trips, several have been cross country. Whether for the two of us, four or five of us, smaller than 27 feet would not work for us. Yes, you could make 24 feet work for you, but it will be real crowded with all of your grand kids (IMHO).
Good Luck with your search.