I think you need to decide is this is for you, or for the kids, and which situations you are thinking of, as I think the advice differs based on that.
My dad boxed, I did TaeKwon-do (nowhere near black belt), my 8 year does JuJitsu as above - for my son I still think it's the right choice over the others. I want him to be able to get out of one on one situations, gain confidence and defend himself without needing to hurt his most likely attacker, another child of a similar age. He should at this rate get his blackbelt not long after starting secondary school which I think will help perception wise for him. When he is older and more mature I will think about boxing or maybe some other ones.
As an adult you can argue things like Krav Maga are best, or MMA, but again it depends how far you would go personally defending yourself, and what your motivation for doing something is. Personally Taekwon-do was enough for me - a Thai Boxer or Krav Maga may well of destroyed me, but how likely was I to face one? I just wanted a bit more confidence, I got it, and that did the trick for me. Everyone is different - just pick something you feel comfortable with and get your kids the streetwise advice they need. Confidence and a bit of know how will go a longway - if you get attacked by a gang with knifes you are basically in trouble no matter what so it's not really something I concern myself with other than doing my best to avoid such situations.