Cassian likes having a plan, Jyn knows that. She knows having something to focus on, something concrete to do, will help pull him out of whatever guilt spiral he had fallen into.

"Do you think you want to give Rey a bath, too?" She probably doesn't need it, but Rey loves the bath; she loves splashing and wiggling and pouring water from one cup to the next to the next to the tub. Maybe seeing his daughter so easily entertained will do more to sooth his frayed nerves.