Yeah, no. Your only argument can be 'it's diiiiiifferent in this isekai 🤷*♀️', because by all other accounts it's inexcusable.

She's not that young, at least elementary school age. Undressing a stranger girl of that age without asking, then even 'helping her wash' is super gross. God, now I sound like a total sjw

Here's how it'd be done:
- ask Rita for help
- if Rita's not available, wait until she is. Latina won't die from 2 more hours of smell
- if Rita died: ask Latina if she needs help
- if she says no, go away
- if she says yes, read the mood and behave accordingly. DO NOT just start rubbing her body 🙈

That's proper protocol. Again, the whole 'he views her as a child' goes counter to the eroge designs. Anime should look at Pocco how a child is done well. Latina gives me vibes if 'there will be a time skip and then hero will marry her'. :/