A.K.
Honestly, I think you are overreacting. There is no doubt in my mind that it was an accident. You shouldn't be mad at that boy...he'll never know to apologize for stuff unless his mom or someone else tells him to. His mom doesn't sound like she's very responsible, but you'll have to just get used to that. There's nothing we can do about irresponible parents, and believe me, they're everywhere.
If you're really concerned about your children getting hurt at such a young age, don't take them to that park when bigger kids are there. One thing I would suggest is to take them when school is in - you'll find it's mostly preschoolers at the parks in the morning and early afternoon. It's very hard at the ages of your boys - I remember the days. Good luck with #3.