I once spoke with an Orthodox Priest who mentioned the difficulties that he was having while trying to baptize a two year old.
Well, you know how stubborn a two year old child can be! Almost as difficult as having to wash a domesticated cat!
When it came time for the baptism, the child would have nothing to do with it. Try as the Priest could to immerse the child, he first clung onto the godparent, and then he grabbed the priest's robe for dear life and would not let go. After spending about 15 minutes soothing and calming a frightened child and with a banquet already being cooked in the parish hall, the Priest had the godparents lean the child's head over the water, and the the Priest poured copious amounts of water over the screaming child's head. To this day, that child hates that Priest!
Probably, in that kind of a situation, with a stubborn two year old, it might have been better to do a rehearsal to see how things would work. After all, a couple getting married has a rehearsal.
Prior to the baptism of my two grandchildren, the relatives practiced with the children to help them not be afraid of the water. In fact, they were very happy to be baptized to the relief of the Priest.