ok the fee is only for emergency exit rows? So if I want to sit with my kids in a non emergency exit row then I don't have to pay a fee?
No, as has been stated the fee for emergency row seats is $15 and the fee for non-emergency row seats is $10 (both prices are per direction).
Not paying the fee does not mean that you will not be sitting next to your children, it just means that you will have to choose from the "left over" seats at the 24 hour mark.
If the flight isn't very full, you should still be able to find seats together (though finding three seats together may be difficult even if the flight isn't totally full).
If the flight is full and a lot of people choose to pay the fees (or if you are unable to check in near the 24 hour mark and many other people have checked in before you) you may not be able to find seats together (or even near each other).
Depending on when your flights are (i.e. how full they are likely to be), how much you want/need to sit beside your daughters, and how close to the 24 hour mark you'll be able to check-in, you can choose to pay the fee (and select your seats now) or not pay the fee (and risk sitting apart).
ETA - My experience with WJ with flights around Christmas was that there were not even any pairs of seats available right at the 24 hour mark. On other flights (not at busy times and not to Florida), there were at least a few pairs of seats left. We were adults who were fine sitting apart, so we were willing to take the risk (and emergency row seats weren't available by the time we booked, so that wasn't an option).
