Telekinesis and necromancy take actual thought to be particularly dangerous, so they wouldn't be too bad... for a child under 2 years old. Telekinesis in particular would be horrible with 2 and 3-year olds.
Super strength could cause a fair amount of damage, but the limiting factor is that it would still scale with both size and strength. A baby 10x stronger than normal still isn't very frightening, since that still leaves them weaker than an adult, and with barely any reach, the chaos would be limited.
Fire and Electricity would easily be the worst. Even at low output, they could start an uncontrollable blaze; they prevent you from picking the child up; and to top it all off, they don't take much thought to use.
Some others:
Invisibility, speed, and flight would be annoying, but it is just a matter of setting up barriers first. It doesn't matter where they go if the can't reach anything dangerous.
Phasing through walls would be particularly troublesome, since you couldn't rely on barriers to stop them; you would need to monitor them constantly.
Teleportation would be much the same, though once again the need for actual thought would likely limit it to line-of-sight, taking the edge off of it.
Multiple powers:
Phasing through walls + invisibility + speed
You can't contain them, you can't track them, and even if you find them, you can't catch them!
