Absolutely a safe place to sit. The case is grounded and the live parts are inside. I’m an electrician and in over 35 years have never seen anyone shocked touching the outside of a transformer like that one.
That’s right, the kids are in very little danger. However, the idiot adults who are standing and walking all over the cables surrounding the transformer deserve what they get. (I have seen people electrocuted while doing that.) The whole area should be cordoned off and the cables covered.