In a way that's exactly what happened in the land before time IV. I don't think that just "friendship" desribes the relationship between Ali and Littlefoot. Otherwise there would be no difference between the "friendship" between Littlefoot and Ali, or Littlefoot and Mo, or Littlefoot and Cera, Ducky, Petrie, Spike etc.
Personally I like the idea of the romance between Littlefoot and Ali.
However, they are both still kids and their kind of love is different from the love between grownups. In Germany this would be called a "sandbox-love", a love between kids without any sexual ulterior motive. This love is very frequent, and maybe one could even impute a less intense variant of this kind of love to Cera (notice she gets ALWAYS jealous if Littlefoot is fond of anybody or at least expresses her dislike of whomever Littlefoot is fond of).
I do not think that Littlefoot and the others even know about sexuality thus far (e.g. the talk about Babies between Littlefoot and his grandparents in LBT 2, or the fact that Littlefoot was not aware that he has a dad in LBT 10), and they probably would have to grow up to develop that kind of maturity. As I'm a strict opponent of the idea of letting the LBT characters grow up I also have to oppose the idea of a relationship that goes beyond that "sandbox-love" described above.
They are certainly going to keep any sexual content out of the land before time. Although I am somewhat surprised at how many parents are absolutely intolerant about their kids watching anything that has to do with sexuality while they have no problem with their kids watching horrible violent movies, I guess I suport the idea of not going beyond that "sandbox-love".
There are land before time fans who thought a lot about an actual sexual relationship between LBT characters, especially Ali and Littlefoot. I know that they wrote fanfictions, and drew pictures of this. Many of these fans are frequent writers in forums and they just take part in discussions like everyone. This is perfectly fine with me. So long they don't put their stuff up in public forums (thus enforcing that stuff on people who don't want to look at it) that are not created for that kind of stuff I do not object anything that is their private business.