I'm so sorry @chocolateanddietcoke that that has happened to your gorgeous sounding little one. I am not a modern mum with experience of such things with nursery age children, although I do have my own grandchildren that go to a nursery school. However, I have read here on Mumsnet, and heard about this type of thing on other occassions.
What strikes me as completely out of order is that the first you have heard about both occassions is from your Dear Son telling you? Is that really correct? At least one member of staff must have been aware of the latest incident as they changed his wet trousers. If they haven't noted it down in an incident and accident book then I would be very worried about the honesty of the staff there.
If your DS was very upset about being pushed over into the puddle, then my opinion is that the staff should have rung you up as soon as they realised he wasn't being consoled and calming down. By that I don't mean that you should be expected to drop everything and go and pick him up, especially if you are at work, or somewhere else where it would have been difficult to leave immediately. But, in the circumstances where my child, or even now, my Grandchild, was practically inconsolable, I would want the choice, and I would want the chance to speak with the member of staff who was going to give my child some one to one time while they calmed down.
At the very least OP, you, or whoever picked your child up from the nursery, should have been told about the horrible incident, and had all the circumstances explained to them. If the push, and subsequent fall into the puddle, wasn't witnessed by any of the staff, I would want to know why not! Our children, and our grandchildren, and indeed any and all of the little ones there, from the 'village', should be being observed, especially when outside, even if the outside area is considered escape proof, and entering into the outside space from outside of the nursery, proof, as well.
I would be calm and light with my child if they had been through such an experience, as I would want them to not feel even worse about what had happened, but I would have hugged them - as I am sure you did - and reassured them that they were right to tell you, and to always tell a member of staff as soon as possible after any such incident. I would probably tell my beloved little one (or even big one) that I was proud of them as well - but then it is possible that in still telling my adult children about twice a year, and my grandchildren more often than that, how proud I am of them, that I might over do it a bit!
When my child was happily getting on with having fun, or had gone to bed, I would give into the fury I would be feeling towards the nursery staff for not telling me themselves about the what had happened - especially as it had meant them having to change his trousers on this occassion. My upset wouldn't be per se about my child being hit or pushed, it would be because they hadn't informed me about it, and also if they hadn't even seen it happen.
When I started this you hadn't had any other replys, by now you have hopefully had a lot. Good luck 💐