You can use rider swap to your advantage and maximize FP+. Your Husband and son book Splash. You collect a rider swap for Splash when they ride. You book FP+ for something else with rider swap, Thunder for instance. You ride solo, if willing, but also collect a rider swap for Thunder. Your Husband and son, or you and your son can then ride Thunder with the child swap. Your son ought to be able to re-ride Splash with you using that child swap ticket collected earlier.
From what i understand child swap tickets are not time window specific, meaning you can use them anytime that day. I have seen examples of being able to save the child swap ticket for a near future date. So if you were returning to MK a few days later you maybe be able to save them for use then. The ticket will say.