Being in the world but not of the world

How do we keep our children PURE and yet let them have friends.
The other day we were at the park and there was a bunch of children, obviously from a local football team having a bbq together. Watching the children I noticed that they were separated into different groups. One lot of boys were off playing football together, another group were playing on the playground equipment and another pair of boys would sulk along on the edge of the largest group in the playground. A few incidents happened that left me feeling a little deflated.
1. The two boys when they thought I couldn't hear them were teasing my two year old daughter about her t-shirt that said "I love Dad". Maybe they were uncomfortable that she was staring at them to get them to get off the swings so she could have a go but I just didn't get how children could pick on a child so much younger then themselves.

2. The group playing on the playground equipment were playing a rough game and one boy got injured. I had no idea as to which child was responsible but what was crazy was that about 4 or 5 boys just up and ran away saying 'it wasn't me, I'm getting out of here'. One girl offered him a free ride next turn but it was strange that no one seemed to comfort him.

I know that there is a lot of personal history and reasons as to why each of these children acted the way they did but it certainly shocked me and made me wonder how are my daughters going to be behaving in another 5-9 years on. How will they treat people?
It is never to early to teach children about being loving to all of God's creation and it is never too late to teach children that they are apart of God's creation and that they need to act accordingly.