Formal:
* Il est temps de prendre un bain. (It's time to take a bath.)
Informal:
* C'est l'heure du bain! (It's bath time!)
* On va prendre un bain? (Are we going to take a bath?)
* Allez, au bain! (Come on, bath time!)
You can also use more specific phrases depending on who you're talking to:
* Bébé, c'est l'heure du bain. (Baby, it's bath time.)
* Tu veux prendre un bain avec moi? (Do you want to take a bath with me?)
The best option will depend on the situation and your relationship with the person you're speaking to.