I think many people have wondered that, it is certainly curious. It's quite possible that the two words were mixed up in the early languages, and that cild took its z-stem inflection from *cind. But that doesn't explain the i, really. For eht > iht it's a regular change, but there is no parallel for elþ > ild. The closest we can get is gieldan from Template:termx, but that's -ie-, not -i-.