Asalamualaikum wrt wb,
All praise be to Allah, and may peace be upon His final Prophet.
Here is some advice I heard from the scholars:
1.) Games and sports are permissable actions, as long as they don't distract a person from their Salat and remembrance of Allah, and they are free from prohibited actions such as cursing, fighting, unlawful dress, and gambling, etc.
2.) Sports can be a means of gaining closeness to Allah if they are done with the intention of worship and making one strong so that they can perform Ibaadah. They kids should be taught gently that whatever sports they play, if they follow Allah's rules, such as being kind to others, and increasing love and brotherhood between each other, it will be counted as a good deed for them. However, the key is the intention.
3.) Here is a good website with lots of Islamic E-books. Perhaps you can find a book and share a short lesson or story with the kids after each game. For example, 40 hadith on the Islamic personality:http://www.islamicbulletin.org/services/all_ebooks_p1.aspx
The kids should be reminded about guarding their chastity, and staying away from Zina, and Islamic dress should be enforced during the games.
Here are some more rulings regarding sports and leisure:http://www.islamqa.com/en/cat/366&page=0
Barakallahu feeki, and may Allah reward you.
And Allah knows best.