Wow there are some deeply unpleasant attitudes on here!
Don't let kids approach other peoples dogs - end of.
Can't control your child, use reins.
If you switch out 'dogs' in this scenario for 'traffic' or 'fire' is the 'well children sometimes do unpredictable things' excuse ok?
Ultimately if you fail to control your child and they run up and get bitten in the face, who suffers the most there?
It isn't the dog owner and it isn't you, it is your child.
Are you all really suggesting that the moral high ground of 'my childs rights trump those of your dogs' is more important than your childs safety?
For those saying they teach their child to ask before touching...
Please don't. This sets up the expectation that they may approach, ask, and be given permission to touch.
It is FAR better if they are taught that they may NOT approach, may NOT touch, unless they are invited to do so, and their parent is present.
For older kids, add on an extra level of safety, get them to look at the DOG.. does the dog LOOK like he wants to be touched, there are body language diagram sheets with simple explanations to avoid dogs with certain postures/expressions.
Dog owners do need to be responsible, I teach this (and the above stuff about kids not asking and assuming they cannot touch) at puppy classes, but it absolutely goes both ways.
If you can prevent your child running into the road, falling into a river or sticking their hand in a fire, you can prevent them running up to an on lead dog minding its own business.