Put a litter box in the tub.  MLB

1. Rinse the tub out when Kitty uses it as a toilet - during the act, if
you're quick enough to catch her in the act.  Should Kitty also happen to
get rinsed a little, she will perhaps get the idea that it is a good place
to avoid.

2. Finding a few inches of water in the bottom of the tub will act as a
deterrent to future such visits. (But watch the toddler!)

Yep. Grease the inner surface of the bathtub, wait till the little f.cker
climbs in, then proceed to fill it with water. Should it look like it's
escaping, hold its head down.

Perhaps she is anxious about using the litter box...... is it in a busy
traffic area?  Cats can get very anxious if they feel "exposed" when they
are using their litter tray.  She may feel "safer" in the bathtub.

You mention that you have a toddler..... the little child may have
interfered (without meaning to of course) with the cat while it was trying
to use its litter tray.

There is usually a reason for a change of behaviour like this.  Perhaps you
can put the litter tray(s) where the cat can get at them and the toddler
can't.

We have our cats' litter tray in our bathroom, just near the toilet.  To
stop our Jack Russell dog or any visiting little kids frightening the cats,
my husband has put a brass cabin-hook and eye on the bathroom door.  This
holds the door open wide enough to give the cats access, but it's not wide
enough for the dog (or little kids) to squeeze through.  That may be one
solution.

I'm sure once you have had a think about why this sudden change of behaviour
has happened you will find a solution.

Cheers!
~Roberta~

Do not keep the litter tray too clean and if the kitten is ever walking
around in there run the tap and it should scare it away, the other
alternative is to put the tray in the bath but that will not really solve
the problem.  I have had the same situation.