I wouldn't do that after only a couple of days. It's not uncommon to have no noticeable difference after only a couple of days, and if you open it to repitch, you'll lose what little CO2 you have already. It's not a "terrible idea" per se because you're not going to ruin it or anything, though, but soda making, like homebrewing, requires paitence.
By the way- I bought the yeast for my first batch of root beer from a beer store that had stopped selling homebrew supplies about a year ago and had a few things left over. The yeast was dead, but I waited a couple of weeks to be sure. I repitched, and everything was fine. In other words, even if the yeast is bad, it should not be a lost cause if you sanitized well. You may have to buy soda for the party, though. If it's not up to par and refrigerated with sufficient settling time, you'll get a somewhat flat or unsettled soda, which may not be worth your efforts.