It's strange, at some times it sounds like Cera, but at others you'd swear it was Littlefoot.
Well I know the two do sound alike as when I was listening to "Always There" I thought at first it was Littlefoot singing in the beginning (I haven't seen that movie in a while so...) but when I heard her say "Almost" lol I remembered that and I was like "oops"
. Then when I heard Littlefoot I'm like "Now THIS is Littlefoot"
Also Littlefoot did at some times have a very feminem sound to his voice...
Thomas Dekker could have also raised his voice for that particular song, and made it sound more magical
And also, why would Cera and Ducky singing make any sense at all in the first movie? Even though (as you said) she was the least likely to join the group, I think it would make a little more sense for Littlefoot and Ducky to be singing as they were the first two to meet up. Also Ducky mentioned that she lost her family in the big earthshake, just like Littlefoot.