If you have not already discovered the Hawk Walk thru the School of Falconry at Ashford Castle, do check into that. While I highly recommend at least one night's stay at Ashford (even better if you do two, as it would allow for fishing time on the lake, biking, etc., but you do not need to stay at the castle in order to book and enjoy the Hawk Walk.
After a brief orientation and short info session re: hawks, each person takes his/her hawk out on a walk thru the woods (with a wide paved walkway for part of it) to launch and land hawks. It is an incredible experience. You can Google: "Hawk Walk Ashford Castle" and view some of the YouTube videos.
If you have not already looked at Ashford Castle as a potential place to stay, check out its web site, too. They run specials from time to time, and it could be worth calling (or emailing them) to see if they might have a special rate for the time you will be in Ireland.
When we were in Ireland (about 10 years ago), we spit our time between B&Bs (some relatively modest) and (grand) castle stays. We also recommend Dromoland Castle. The Old Presbytery Inn (10 years ago) was still configured as a regular B&B/inn at that time, it was lovely and kicked off our Ireland visit beautifully (our first night), but I see it is now set up as apartments (likely even better for your family). It was wonderful and nicely located, and if the owners are still the same family, they are incredible hosts.
Lucky sons you have to enjoy a special vacation like this.
EDITED to add: If your budget allows two-night stays at castles,I highly recommend that vs. a one-night stay. That way you get settled in and have time to soak in the environment and enjoy the activities of the castle grounds/lakes without having to rush to checkout the next morning.
EDITED (again): Sky Road in the Connemara region is listed in National Geographic's Journeys of a Lifetime book. It is a lovely drive and would put you up near Ashford Castle in Cong. If you stay more to the South, do not miss Connor's Pass (I would have to look it up on a map to recall exactly where it is), but lovely.
If you decide one vehicle is too tight for your family, you really might consider hiring a driver who has a van. Even though there were only 2 of us when we visited, we did that, and when it was all said and done, I analyzed cost vs. if we had rented a car, paid for petro, the add-on insurances (that you DO want in Ireland), and we came out about the same. But, the days we did two-day castle stays, we did not have the driver...he dropped us off and picked us up, but he took other fares during those days. It was nice having a local drive and also tell us a bit about Ireland while we comfortably looked at the scenery, not worrying about where to turn or how to navigate sheep in the road, etc. I am not sure the driver we used is still driving (last time I emailed him,it bounced back), but you could ask B&Bs or inns for recommendations. Or, as I say, you do not need to all or none...you could rent a car for part of your vacation and hire a driver for other parts........whatever works for your circumstances.