But on the other hand many of the LBT stories could take place in a very short time span. LBT 2 might begin just a few days after the original story (which is likely considering how much they celebrate that they "...finally found a peaceful valley". There is nothing that would disprove the assumption that LBT 3 takes place few days or weeks after LBT 2. After LBT 3 there must be some gap as it certainly took a while for the Great Valley to recover. But LBT 4 may again be followed by LBT 5. After the fifth movie there must be a break again (not as long as after LBT 3 though) for the leafs to recover. LBT 6, 7, and 8 May again come in close succession. It's fall in LBT 6, and it may have been in LBT 7, so the 8th movie could follow few weeks after. Once the snow melts (which could produce a lot of water to support the flooding of the Great Valley) LBT 9 could begin and again LBT 10 could come right after it.
As Apatosauruses grew by 2kilos a day (according to "Walking with Dinosaurs") Littlefoot would usually be quite huge already. I don't think that they are going to change anything about the character's outward appearance, though I could imagine changes in their conduct. But with really young kids being declared THE target group of LBT by Charles Grosvenor I don't suppose there will be anything serious, as they seem to assume that THE target group can't handle as much a sharptooth like the one in the original movie. :mad
Still in general I really prefer the characters the way they are. I would not approve of a Bron-like looking Littlefoot, a Cera who begins to resemble her Dad, a Ducky who resembles her mother etc.
As for an LBT 13, I don't know of any official information. I suppose that whether or not there will be a 13th movie will largely depend on the success of LBT 11 and 12.