As a huge fan of Lego, I think this would sound like a rather neat idea. However, I must admit that I have doubts it could ever become an actual product.
First, we would have to consider what the idea would be. That fact that Land Before Time takes place in a world of dinosaurs with no human influences means there are very few possible options of what could be built out of Lego parts. I honestly have a hard time coming up with any ideas for playsets (hardly anything more than just rocks and trees), so perhaps it might be better to have the characters themselves be built out of Lego, possibly even with articulated limbs for added playabilitiy. (Keep in mind, Land Before Time is considered a kids franchise, and Lego is a kids toy, so of course playability should be considered.)
Also keep in mind that new molds are not permitted for any sets that come through Lego Ideas, so whatever we come up with has to use preexisting parts (although options for colors aren't as restrictive). We should also keep the number of pieces in mind, because most Lego sets tend to be priced at around 10 cents per piece, so a set containing 1000 pieces would retail for $100. If you want this idea to not only become an actual set but also be rather affordable, then it would be wise to use the fewest pieces possible.
Even if we do come up with a fairly good idea, there's still a lot of other factors that make me have doubts. First is that an Idea needs to reach 10,000 votes within a 12-month period to be considered for review, and let's face it, Land Before Time is hardly a popular enough franchise to receive that much support. Even if by some miracle we do manage to reach the 10,000 votes, that's still no guarantee that it will become an actual Lego set. I've heard of many interesting ideas that reached the 10,000 votes, but did not receive final approval.
I'm not going to help design such a possible Lego set (I've got other things I'd rather spend my time on), so whatever advice I can give here will have to do. I have my doubts this will be successful, but if you want to give it a try anyway, then go ahead and go for it.