The translation in the exercise is different from the one above, it translates to "I need to help myself". But "kujisaidia" really means to go to the bathroom.
Is "hivi" just part of the phrase? Would it ever be exchanged with another demonstrative?
"Bathroom" is United States English for "Toilet". "Lavatory" & "loo" (among other possibilities) are also possible in England.
No, you could use "toilet" or "loo" for "choo" - however, "kujisaidia" is to avoid using that (it's a taboo/used in the family or by small kids); just as you would usually ask for the bathroom or lavatory in a restaurant (not the loo).
Actually, I would never ask for the bathroom in a British restaurant. They would wonder why I wanted to take a bath.
I think BwanaSimba is referring to the translation given at the top of this discussion page.
As explained above, "choo" means toilet, but it is impolite to make a direct reference to the toilet when you need to use one. You use this fixed expression "I need to help myself" instead.