手伝い means assistant, and I was tempted to use that instead since it fits for Nitori... but I think "little helper" carries the same connotations, sounds cuter, and conveys what she's actually trying to say a bit better.
Interestingly, いる can mean both "to be" and "to want/need" depending on the Kanji -- but Kanji isn't used. So, whether she's saying "Will you be my little helper?" or "Do you want a little helper?" is ambiguous.
I feel like it depends entirely on what it is she's holding. It might be related the the object in post #2178155. Maybe she means both things. Mutual assistance.
However, at least one person on Nico thinks it might be a teacup.
Fairly confident that the use of いる is the "want/need" connotation. "手伝いいる?" with the "be/exist" meaning would translate to "is there any assistance?" which doesn't make much sense. Now that we've established "want/need" as the intended use, the question is does SHE want/need something or is she asking if YOU want/need something. Given that her statement ends with a question mark, the latter would fit much more readily than the former. If the intended meaning was "would you help me?" I would expect something more along the lines of "手伝いをしてくれない?" or something using ほしい。
Thus, I would probably interpret this cute lil' kappa's words as "d-do you need any help?" (Yes, please). So she is indeed probably holding an onahole of some sort.