Also note that there are no longer any trundle beds at POR. They are now Murphy beds. You can check out the photos on the POR website.
I agree that WL would be the handiest resort to MK, if the majority of your park days will be there. Otherwise, POR/POFQ are probably the most central of all the mods, transportation wise.
Also, at whatever Disney resort you select, you can ask for bedrails for your child's bed. I don't know about the Swan.