As it currently is, we have to make a strength/dexterity check to prevent us from being knocked down, and then make a balance check to see if we stand up. Now, I'm not gonna say you have it totally backwards, just, almost completely backwards...
If somebody tries to knock you down, you need to keep your balance to keep from falling/being knocked down, of course, your strength and dexterity helps with that, but that's why there's a dexterity modifier on the balance skill right? So, make that add both your strength and dexterity modifier to your balance. Then make it so your balance determines if you get knocked down or not, and finally, make it a dexterity check to see if you can get back up, or, for the better, more sensible, make it so you always get back up within a set time. Because, let's face it, the only time your not going to get back up when your tripped/knocked down, is if you are either dead, or disoriented from being knocked down. At which point, your dexterity could help you get reorientated to get back up.
Also, the only times I can see where your balance would be useless to see if you got knocked down is if you get knocked off of your feet, but you can replicate that via super high save, which a super high balance would work against, because you can land back on your feet.
So:
-balance check to see if you fall down.
-get back up after 1-2 seconds
-unless it's an especially disorientating fall(special knockdown effect, dexterity check to prevent disorientation?), then it takes a dexterity check to reorient, as well as a timer for the especially clumsy.