If playing with regular cards, remove tens and face cards. A great way to reinforce place value!
If you don't feel like separating the cards, children can add the value of the two cards flipped (all face cards=10; or jack=11, queen=12, king=13). Child with the larger total collects the cards.
Can also subtract or multiply the numbers from the cards flipped. Added challenge: add, subtract, or multiply more than 2 cards each turn.
For middle school, try using 2 cards each turn to create the larger fraction: