Being able to pre-select your seat is not necessarily included with your air tickets these days. What are your options?
Don't assume that you will be seated together just because you are a family.
You must be sure to request your seats at the time that you purchase your tickets. In many cases (e.g. with every "charter" air company flying out of Canada) there will be a charge for this from the airline.
If this is a last minute purchase, you may not get what you want and, indeed, your family may be scattered all over the airplane. If you don't pay for your seats, you need to be aware that this is a possbility. This is nothing new, by the way; there is no "law" (and there never was) requiring children to be seated with their parents...
There are a couple of "free" options, but they are somewhat limited, and if you are risk-averse you may not want to chance it...
Nearly all airlines now allow (and encourage) you to use on-line check-in. As part of the check-in procedure, you will be given the chance to select seats. You can only check in online within 24 hours of travel, though, so you need to remember to do it in time. There are a limited number of seats held for on-line check-in.
The old-school way still works, but you need to get to the airport EARLY to give yourself the best chance -AT LEAST 3 hours for a charter flight... As the airport seat allotment usually is the same as the online check-in allotment, it is still possible to get to the airport and not be able to be assigned seats together.
On the plane
Usually on the plane the flight attendents will attempt to get families to be able to sit together, but it may not always be possible. This can be very stressful and upsetting for some families. And think about it - people that DID pay for their seats selections are unlikely to give up their paid seats just because you didn't feel like paying extra for yours! We're just sayin'
Booking seats in advance
Charter flights have made it possible for you to request seats in advance but they charge for this service. Presently the charges start from $15.00 per seat per direction per passenger. For a family of five this means an additional $150.00 minimum for a return flight to a sun destination. This is an airline charge, not an agency charge and must be paid for by credit card.
Some charter companies offer Club Class seats... similar to Business class... on flights to certain destinations. This upgraded seating goes for about $175-$300 per seat per direction (depending on the airline) and can be well worth the price on long flights and for people who have problems dealing with cramped seating.
People travelling with small children like to request bulkhead seating (this means that there isn't a row of seats in front of you). If you are travelling with a very small baby (6 months or less) you can often request a bassinette (sky-cot), which is only available at these bulkhead seats.
Children, the physically handicapped and some elderly people cannot be seated in exit rows. This is due to the fact that they may not be able to remove the exit doors in case of emergency. Airlines usually reserve these seats to assign only at check in time, though at least one commonly used charter airline lets you choose an exit row (for an outrageous supplemental cost!).