Hi Amanda,
For families, you have a few good options for hot springs in La Fortuna/Arenal. Baldi has a separate children’s area and also a huge slide (slide is better for older kids). A little nicer is the Springs. They have a twisty slide that is probably too much for kids your age but you also can visit their wildlife sanctuary as part of the cost of admission. Another idea is Kalambu, which is more of a water park.