Hi K.,
We went through this with my son and my daughter.
At some point for some reason they just suddenly have an aversion to all kind of things that they loved before - who knows why? But it is most frustrating and usually passes and is just a phase.
For my son: the solution was bubbles! We introduced bubble bath which he did not normally have and so it became time for "bubbles" versus "bath".
My daughter just needed some new toys and a bit of fun time running around in prep for bath (usually naked!)
Filling the bath with the shower head (the type that can be hand held), also introduced the concept of a "tickly bath" which both kids like. It also makes a lot more bubbles and also clues them in to help clean the bath at the end, because this is fun.
Sometimes they like to bath alone and sometimes together - and trying to work that out can be a bit of a challenge. Sometimes they like Dad, sometimes Mom, so a different person (if available), may work too.
Trying to find a new way to make it fun, seemed to do the trick. We have a few old shampoo bottles for filling and emptying. They bubble up under the water and that seems to be entertaining for all ages. We put part of my daughters tea set in the bath and they both love to pour and fill and empty and serve bubbles.
My son was fine with anything but I had to switch bubble baths for my daughter as she has sensitive skin. Now I have to use Rainbow Organic Herbal Bubble Bath for Kids which I get from Whole Foods.
Hope this helps.
S.