That's a bit silly - instead have a mercy rule. Once a team gets, say, six goals ahead then the game is over. If there's still time to kill, swap players about to make a game of it so everyone gets to experience a balanced game.
Ugh... that's wetter than freshly washed lettuce... no way kids are going to swap teams... huge parts of it is to do with taking part and team spirit and being in it together... Lets stop babying kids to the nth degree.... part of the joy of success is to have experienced the other end of the spectrum.. you cant just pat them on the head when the going gets tough and say never mind I'll even it up for you... these are life lessons.. they are going to encounter adversity as they go through life, they are going to face days when they will be demoralised and feel like chucking it in.. but there wont be anyone there to rufflle their hair and make it a bit fairer and easier for them..
That's why we have a generation of FB posting sympathy vampires crying and whining every five minutes because there lives aren't perfect.. life is tough and instilling mechanisms to combat this is important otherwise you are leaving them mentally defenseless