Not true. For quite a few years now charter airlines have been charging passengers to pre-book their seats (often you cant actually choose exact seats, just that you will be 'together' on the aircraft)
Its purely optional, of course there is a small risk that many people will do it and you won't get seats together but very unlikely if you arrive in good time at the airport.
Also depending on the ages of the children they may HAVE to seat a child with an adult anyway.