Just back from my 7th or 8th visit. Without a few more details its difficult to guage what sort of cost you are estimating, and what you consider good value.
My experience has been that going with
www.mytravel.com out of Belfast by charter is probably best if travelling with children, direct flight are preferrable to changing planes somewhere. I'm not sure which tour operators operate charter out of dublin, so can't really advsie there.
Most charters offer car hire as part of the holiday, but be aware that taxes / insurance are on top and can easily run to a couple of hundreds of Euro per week.
For 5 I'd consider a villa, as it gives you some flexability and you can make basic meals during your visit (you do get tired of eating out all the time).
I'd look at maybe doing 1 week in Orlando and another week by the coast, be it Clearwater / Naples / Fort Lauderdale, as theme parks are exhausting and get very expensive.
Look at buying tickets for parks online before you go, there are normally savings to be had there, and decide early on which parks you really want to see. I love universal, and would return day after day, so a 7 day ticket suited me, but disney i've done before so a days ticket to Magic Kingdom suited me.
most places around International drive / Kissemmee are handy for seeing the sights, and a car is pretty much a requirment, even if your hotel does a shuttle.
You should be able to get a quote from mytravel.com now, if you want a starting figure.