3 possibilities: 1 targeted attack, 2 Russian missile went off course, 3 Ukrainian counter-fire missile fell on the hospital.
A few basic observations:
1. Russia gains nothing from an attack on a children's hospital and the resulting casualties - only the opposite as the resulting NATO propaganda machine hypes it up the wazoo. It makes absolutely no military sense to shoot yourself in the foot in such a manner. If one says it was to intimidate or demoralize Ukraine, I would point out that this only makes sense as part of a sustained strategy, not an occasional strike or one-off. So when this happens 100X in a week then we can talk about that idea, but otherwise it's not reality.
2. Military Strategy Channel over on Youtube identified 3 potential targets near the hospital - an industrial complex 1300 meters to the north, a govern aviation ministry building just down the block from the hospital, and an aviation college building a kilometer or two to the south. So it's possibly an off-course missile or one damaged by counter-fire could have caused the hospital hit.
3. A number of times NATO/Ukrainians have screamed about supposed "attacks on civilians" (or on Poland in one incident) from Russian missile strikes, only to have it turn out to be from falling Ukrainian missiles fired in failed counter-fire. If I had to guess, this would be my first choice just based on the prior track record.
I suppose a 4th option would be a deliberate false flag by NATO elements, but I won't go there...