As others have said, there are reserved areas for the parades for wheelchair users and their party. They are all marked on the regular park maps with a wheelchair symbol. Unless the area you're trying for is particularly busy, you should be able to keep the whole group together, with the rest of the group sitting behind your father's wheelchair. There are also wheelchair viewing areas for Illuminations and all sit-down shows including Fantasmic. These areas can all fill up fairly quickly, so you might want to stake out your place reasonably early.
For MK parades, I particularly like the spot in Liberty Square, opposite the Hall of Presidents. This area is fairly large and not as popular as the hub, so doesn't get as cramped as some others; it has a wall along the back of much of it, so the rest of the party can sit down, and nobody can crowd in behind you; and for the daytime parades, you get shade from the liberty tree and air con from the Hall of Presidents.
Wishes, unfortunately, doesn't have any reserved viewing areas for wheelchair users. The thinking is that, as the show is in the sky, it can be seen from everywhere in the park. If you booked through Disney and are staying in one of their hotels, you should be given tickets for a reserved fireworks viewing area, which might be helpful (I've not used it myself, but do be aware you have to book your place in advance).
Otherwise, anywhere where you can see the castle and the space behind it, should give you a good view. The hub and the top of Main Street get very crowded, so we tend to favour the bridge to Tomorrowland. If you make your way there as soon as the parade finishes, you should be able to get a reasonably good spot. Try and grab a spot on the bridge closest to the hub; that way you can get a good view of the fireworks, and Tink flies pretty much right over you. If you're at the front against the bridge's railing nobody can stand in front of you; but if you're feeling assertive, you can often get a better view with your back against the other set of railings. You may need to keep asking people to not stop in front of you, but you don't have to worry about the trees so much.
You might want to consider renting a wheelchair from a company local to you, rather than from Disney. If you rent in the parks, your father will have to walk around your resort, and between the bus to the park; whereas renting for the whole trip will allow him to use the chair the whole trip. I suggested renting from somewhere local to you, as this is often cheaper than the Orlando rentals, and it will mean he can use it around the airport.
Hope you guys have a great trip!
