I am so fed up with this situation!
Last year I once asked sensei if he couldn't very gently suggest that the kid teacher should, from time to time practice aikido techniques instead of only tai sabaki and invite him to participate again to adult lessons. Sensei said that he had already tried, but without success, and that he didn't want to hurt the kid teacher's feelings (who is normally a very sweet, old guy). I suppose that's why he came up with this "assistant teacher" scheme, but it doesn't work since the kid teacher prefers NOT to have assistants in order to keep to his preferred set of warming-up and tai sabaki exercises instead of teaching ukemi and kihon waza.
This kind of "hinting" strategy is quite common in Aikido (more so than other places I've seen) - but it causes more problems than it "solves". Just tell the kid's teacher straight up what your concerns are (it can be done nicely, but directly). If he doesn't like it, then leave. You don't gain anything by staying and enabling the situation to continue.