Good deal twiles!
I don't know what your pH is now, but use the pool calculator to determine how much acid to use. Check the strength on the bottle and enter it in the calculator. If it is liquid, pour it in slowly in front of your return to the pool. Powder, dissolve it in a bucket first and then do the same. Use care, and wear safety glasses. If I adjusted before swim time, I would give it an hour to mix, or I would get in after 20 or so minutes and stir up the pool myself before putting the kids in.
If you pH is less than 8.2 and 7.2 or above, it's fine for swimming. Optimum is generally 7.4-7.6 for eye comfort. Let them enjoy it today and adjust this evening.