:::delurking here...we haven't switched our cable options to showtime yet so I've been popping in to read what's happening on the L-Word! We'll switch towards the end of the season and I'll catch them from OnDemand.:::
My husband and I have been during Gay Days and have been in the Magic Kingdom for the day everyone was scheduled to be there. My kids are 3 and 5. For us, it was a non-issue. We saw no inappropriate PDA on that trip and I'd agree we saw more from hetero couples on other trips than we did from gay/lesbian couples on this trip. Children are very open and accepting. It will only be an issue if you make it one. My youngest didn't comment at all. My 5 year old commented on a gay couple there with their daughter. We were behind them in line and he heard their daughter calling one of them Dad and one Papa. My FIL is Papa to my sons so he said "that girl called him Papa but he's not old enough to be a Papa" (to him Papa = grandfather and these men were both mid 30s). I said "Papa doesn't always have to be a Poppy (my Dad is Poppy so that's his word for Grandfather). It can mean Daddy too." My son said "but she said HE's her Dad." (pointing to the other man) I said "but she has TWO Dads so she must call him Papa so nobody gets confused." The men in front of us smiled and one sort of mouthed "thanks." DS looked back and forth at them and said to the little girl "you've got 2 dads?" she nodded and he says "oh...that's cool."

He asked later if the girl had 2 Moms too and I said no, that most families have a Mom and a Dad but that sometimes families are a little different. We talked about how the kids 2 doors down live with just their Mom and their Dad lives in another state (divorced) or his friend who has 2 families (parents divorced, both remarried with other kids and they share custody of DSs friend) and explained that in some families there are 2 Mommies or 2 Daddies. Later that day he pointed out 2 women standing with their arms around each other's waist and said "that must be a 2 Mommy family." That was that for him. He's never brought it up again but if he did, we'd answer honestly as we do with everything else that comes up. It will only be "uncomfortable" if you make it so. If they have questions, answer simply and honestly. We had friends there at the same time and they said their kids 7 & 10 either didn't notice at all or didn't comment if they did. Their 14 year old asked that evening if they noticed there were a few gay couples there. They said yes they noticed too. Their son's only comment was that there was a guy in line for POTC who sounded like he was a "Will" from Will and Grace. The only thing very "overt" that we saw was a group of maybe 8 men all with red shirts and Tinkerbelle stuff (pins, hats, red t-shirt with the pic of Tink sitting with her arms crossed that says "spoiled rotten" above it, one wearing a set of wings, one with the wand etc) getting off of IASW as we were getting on...they were singing along and laughing etc. Nothing offensive at all. My 3 year old said "they are being silly!" and giggled. They didn't see it as anything different than when DH was cutting up with Minnie at Chef Mickeys that morning or when he was acting silly with the waiter at Whispering Canyon Cafe.
I'd say don't worry about it. Go and have fun. If you are concerned, look at the schedule for what day Gay Days is scheduled to be in the Magic Kingdom and just plan a different park for that day. I would bet with your kids ages that (at least with the older ones if not all of them) they've seen something on TV with gay characters or have at least heard about those shows from friends. Will and Grace is on in reruns in the afternoon/early evenings now so it's not like it's only in the 9:00 hour on tv. If the kids ask anything it's more likely going to be something that can be answered easily and simply saying something like all families are different or just saying sometimes people fall in love with someone of the same gender. They aren't likely to ask anything that would require a big discussion at Disney like "and how does that make YOU feel" or "and what does the president think about that?" or any in depth questions about their lifestyle. If you feel like you need to discuss anything further with them, you can always offer the simple explanation on vacation and wait till you get home to open up a discussion about anything else.
Hope you have a great time on your vacation!