I believe that this is a matter where LBT generally had to take some liberties about timing to avoid slowing down the plot. Based on LBT 4 one could assume for the land of mists to be just a few minute or very few hours walk from the Great Valley. It is rather unlikely for a place to be exposed to so dramatic climate changes as described in the movie while a place just a short walk away would be totally unaffected. Moreover so short a distance would really increase the degree of lacking concern on the part of the grownups who refused to take a little walk rather than a long term expedition on behalf of Littlefoot's grandfather. Now of course it had been said in the movie that it was a terrible place etc. but throughout the entire expedition of the kids I don't think we saw anything that would have posed a real threat of any kind to an adult longneck (who could have easily dealt with Dill as well along the lines Sue later dealt with a different crocodile). Even the kids felt safe enough to just sleep in the land of mists without even assigning the task of keeping watch to one of them.
You are not going to find an entirely realistic explanation for these and some other points (e.g. how come that in spite of having apparently lived there for a while Ali never actually saw the night flower before), but other than assumptions (like a realistic distance of the Great Valley to the land of mist slowing down the pace of the plot) nobody will know why such not realistic elements exist, but most of us are likely able to go with them.
