This fountain seems to be designed to allow for this type of use (though not specifically for it), .
I find the sight of children playing in fountains repugnant. It's disgusting and trashy. (No sarcasm.)
Please, this [State Street] fountain, and the north end ones are specifically designed for people to be in them.
If you want water, find .. a fire hydrant to open
While people may differ on whether the sight of children playing in fountains is repugnant. Often it is indeed very filthy. Here some food for thought - your much more likely to get water borne infections from frolicking in spray pools than regular swimming pools. In many fountains the water is continually recycled without chlorination. Very young kids stand in the spray pool with poopy diapers, water washes poop residue into drain, water then is recycled along with new additives. That being said, I have heard some new fountains do chlorinate and/or do not recycle water. Not sure about the fountains in the Greenway.
Here are some pics from Saturday June 21st.
One thing...this fountain is designed as a destination and with the exception of the southern boarder, which opens to the road (there is a small concrete barrier/lip...but not much), it is pretty well designed. I'd never compare it to City Hall Plaza...which is not a destination except for sports rallies (gotta love the "jumbletrons") or for food-festivals, neither of which are regular happenings. The people at the fountain weren't gathered because there was an "event' taking place. You really can't compare the two.
However, I do find it pretty darn odd to have a fountain right in the middle of what is basically an intersection. That can't be totally safe with kids running around and whatnot. There has to be some way to enclose that a little...