Based on the circumstances as reported here, the criticism of the IDF missile firing that killed ten people near a UN school is justified. Targeting three people on a motorcycle riding by a compound where civilians are lining up to buy food and other items amounts to an attack on civilians; the missile, according to the linked article, "hit the motorcycle" and then "crashed into the road," sending shrapnel flying "in every direction." The fact that the three people riding the motorcycle were the intended targets (and were apparently killed along with the others) does not matter, under standard notions of proportionality. So the UN Sec-Gen's statement ("a moral outrage and a criminal act") is justified.
(Note: Post edited slightly after initial posting.)
Update: Hank in comments has pointed out that the WaPo article, standing alone, does not provide enough info to determine whether this was a legal violation, because for that judgment one has to know what the personnel in the plane knew about the situation on the ground when they fired the missile. That point is right, although as I say in the comments it seems likely to me, as someone admittedly ignorant of jet-fighter technology, that the personnel in the plane either knew or could have informed themselves about what the situation on the ground was, i.e., that the motorcycle when they fired on it was passing a school with civilians standing outside.