also at any first aid station there are air conditioned places to sit or lay down, if she is getting upset or has heart problems related to DS.
if you choose to use a stroller for her, get a GAC that says "stroller as wheelchair" so you can take it into attractions that otherwise would make you leave your stroller outside. she may do fine for short distances, but waiting in lines where you have to leave your stroller outside (small world, for example, or even the waitroom in philharmagic) may be difficult.
if she gets hot too easily, even look at those little self air conditioning units... it sits on your shoulders and can really help. they are not too expensive at brookstone and sharper image. i know kids with DS can have heart issues, so if she is on meds and cant be in the sun, for example, she can get a GAC for this as well...
check out the FAQ thread!