If you booked through First Choice you would have to transfer to one of their packages. If they do not have a Eurodisney package they can't transfer your money to one. As suggested have a look at Port Aventura. If this is not suitable have a look for a holiday with good child facilities.

You didn't say when you were due to travel long haul. Have you been paying your holiday in installments? Because you've only paid £1000 I presume you aren't at the point in time when full balance is due. If so I don't think they can keep all your monies paid, only the deposit, should you decide to cancel. I'm not 100% on that but maybe someone on here knows for definate.