I believe Japanese children do not use special mnemonics. I mastered the sequences of , , , and using the latter half of this children's song (written by a famous poet), and this was probably when I was a kindergartner. I still clearly remember these five sequences almost like standard words, although I have forgotten the remaining parts of the song. Children can remember something like this fairly quickly, and it's much harder for them to remember a long sentence.
If you are interested in mnemonics for JSL learners, please see Ben's answer for an example.