We sing a blessing before dinner... it's the "blessings on the blossoms" song from our oldest child's preschool program. All the kids sing it together at the preschool before lunch and we adopted it for home use (we're pagan). DD1 is very comfortable singing the blessing, and dd2 (3yo) will ask if she can sing the blessing when we are visiting family who don't do a pre-meal blessing. Since it's a song it's not really a "leader" situation, but both girls can and will volunteer to sing at home and when visiting, and they don't seem to care how many people sing along.
I think a lot has to do with the child and with the type of prayer/blessing. A song might be easier for a young child since so many "children's activities" have little songs and rhymes? Or maybe it would feel easier since it's often a group activity? DH and I want our kiddos to feel comfortable singing in public as well as praying in public so it kind of kills two birds for our family, but either way I could see a short song/poetic prayer being a good way to lead a child into being comfortable with the public speaking aspect.
If it's a question of how much the child "understands" the prayer or something like that... I think that would totally depend on the child (and the prayer, and the purpose of the prayer). I was raised with a christian meal blessing and I always saw it as a formulaic "what we do before we eat" thing rather than a "meaningful spiritual act" thing until well into our teen years. Kind of like having a napkin on your lap or your elbows off the table or thanking the host. lol So even though I was comfortable leading the prayer by the time I was 5 or 6, I wasn't really "understanding" the prayer on a personal level.