this may seem like a complete disaster but i am fairly sure it will not turn out that way.
firstly, the "old" coaches have an entertainment system and they show family type films and there is the option to listen to music or childrens entertainment.
even if you can only sit next to 1 child, the others will be in the row beside you or in the row in front or behind. you are very unlikely to be split up.
i presume that you paid a supplement to go on the new coach and i presume they will return this. i dont know how much they charge for the extra upgrade but the old coaches entertainment system was adequate for the journey.
remember that you have disney to break up the journey, then the regular stops. don't get hung up on not having state of the art entertainment or it will spoil your holiday.
loads of families travel every year with siesta and other companies; some families take the kids electronic toys/games systems to supplement the on coach entertainment and to use in resort.just make sure they are charged up before you go or you have spare batteries.