I'd recommend you visit Epcot as they have great displays and wonderful gardens there. It's not quite so exciting for the kids, but might be a welcome change after all the over-stimulation of the other theme parks.
It really depends who you get as a tour guide, I'd imagine there must be more than one at the HGTV show, surely?
A little drive, maybe 45 min. east from Orlando (and not a garden) there is also the Kennedy Space Center. Kids love that.
In the other direction, north maybe a half hour drive is the Bok Tower gardens in Lk. Wales.
http://boktowergardens.org/
Of course if you're at all interested in orchids, there are many commercial orchid growers in the Apopka area, just northwest of Orlando. A few but not all sell direct to the public, and you can get some fantastic deals. That's always a stop or two when I go up thataways.