Soccer is difficult, because of the size of the pitch. There is no such thing as a universal lens to cover it all. If you try to shoot everything, you may end up with nothing.
I normally chose what part of the pitch I would be looking for pictures from. In your case, I am assuming you would want pictures of your kids, so it depends on which position they play. If you read the game properly, you should be able to get good shots with your 70-200, with the right shooting position. That's the one I used the most. An advantage of these kind of games is that you can normally move freely around the pitch and get the best possible positions.
If you win the lottery and can buy a 200-400 f4L IS 1.4x, that will do a better job for some parts, but you would still need to complement with the 70-200.