While I haven't been to St. John's Castle I have heard it mentioned as one of the highlights of Limerick. Rick's book (2013) doesn't mention Limerick. It did not get good reviews from Frommers. They said it costs more than it is worth. They also said access to the actual castle structure was limited. However, you know your children best and if you think they would enjoy exploring the castle then it is worth a stop and would not be a wasted day for your family especially if Limerick is on your route. When we traveled with our children at that age we once spent almost an entire day at a castle in England. Since there was hardly anyone else around the guides gave our children extra attention and information as our children were intrigued by the whole castle experience. I'd have a plan B in case the castle plan doesn't work as I always felt this helpful when traveling with our children.
As an aside, not sure of your route but Cahir Castle was a fun place to visit and it is not far from the Rock of Cashel. Blarney Castle or Bunratty Castle and Folk Park would be my other recommendations. Both were enjoyed by my children.
Barbara