Cannot believe some of these posts!
OP had her hands full with a clingy, unwell, breast feeding one year old, that was her completely occupied.
Op's husband was supervising the 6 and 5 year olds, in what was clearly quite a big open space with lots of people, they couldn't be left to wander around not being watched! So he was fully occupied with those two. Are some of you SERIOUSLY suggesting 6 and 5 year old should have been left, unsupervised, in a big open outdoor place?!
MIL was occupied with 3 year old, who was running around, wanting to play with 5 and 6 year olds but understandably is too little to do so when some of the play equipment was just too big for her. If as suggested she is a bit "wild" for whatever reason and is used to just running free, and especially as MIL is clearly older, it makes sense MIL was completely occupied with the 3 year old.
There simply wasn't anyone available to appropriately care for the 2 year old, especially if he is aggressive to other children! OP has stated in later posts he was aggressive and violent towards her 1 year old the previous day, it would have been absolutely impossible for OP to care for her 1 year old who would need holding/carrying, be the one looking after all the bags, AND let the 2 year old out, who would likely run off. OP and her husband don't sound like they have regular contact due to distance so were effectively strangers to the little boy which would not have helped. I certainly wouldn't feel confident taking responsibility for a child who is aggressive and violent, not only would i not want to be hit and kicked myself, but i wouldn't want to be responsible for trying to stop them doing so to all the other kids around. 5 children amongst 3 adults simply doesnt work when those 5 children split 4 ways!
The mother of the 2 and 5 year olds was getting her hair done, but only left because she believed her sister, mother of the 3 year old, would be present.
Mother of the 3 year old was apparently sitting in her car in the car park. Had she come to the play area, SHE could have observed/chased after/cared for her own 3 year old, and MIL, who is familiar to the 2 year old, could have cared for him, let him out of the buggy, and played with him one on one.
At the end of the day, this family NEEDS to communicate with each other more. The 2 year old has some additional needs, behavioural issues, or something. Being so violent and uncontrollable all the time isn't normal, whether it is medical or due to neglect, it NEEDS investigating, for the poor childs sake. The mother clearly isn't coping with him, and restraining him in a buggy or high chair all the time, won't help. Mother of the 3 year old also needs some help, being a single mother is hard, if she honestly needs to sit in a car park in her car alone instead of looking after her own child, even when it means the other caring adults are stretched too thin, something is wrong there too. As for poor MIL, she wants to do right by her daughters, she sees them struggle as single parents, gets moaned at how they need a break, and ends up caring for the kids, sometimes all 3 at once, when it is too much for her and at times when she honestly doesn't want to, but gets hostile responses when she voices her desire to have time to herself not as a childminder. It's not healthy for anyone involved, especially that poor little 2 year old boy.