I think it would be perfectly fine to concern oneself with childrens rights in Israel. However, if one were to concern oneself with "childrens rights" on that basis, as opposed to supporting "Jews" in Israel, which seems to be the aim of the UJA "walk for Israel" (lets not forget the UJA is not just donating to children, but also other kinds of aid) then I think one would find oneself spending a lot of time defending the rights of Arab children.
For example, one might defend the right of Arab children not to be excluded from aid being distributed obstensibly as aid to the poor, only because they are Arab and not Jewish, because condoning such a distinction would be supporting Apartheid.