I have 4 kids, so my experience might be different, but I ALWAYS pay for advanced seat selection. It's not that much incremental $$, and it gives you the piece of mind that you will be seated together. There are no guarantees if you do not buy advanced selection. You might be able to get seats together at check-in, but if not, then you have to plead/argue/etc with either the gate agent or fellow passengers, either of which is not appealing to me at all. I see countless examples of families getting into heated discussions with gate agents during boarding trying to get seated together - but I have no sympathy. Everyone has the opportunity to ensure they are seated together by paying for that privilege.
So, yea, I advise paying the extra dollars for advanced seats.
