As an exercise this is simple. As a task, it can be daunting for most people. I do not know if it is a named technique or even if it has been used earlier.
It is a challenging exercise for most people.
Ask your child to write down the alphabet.
MAke sure (s)he can read out the names of the letters (s)he has written. It helps to have them in proper order, too.
Now using this as a refrence grid, the child must compose a passage with each subsequent word being in alphabetical order.
Encourage the children to use dictionary and thesaurus to hunt words with different alphabetical beginnings.
Encourage children to use new and fresh words. Good, bad, tasty should not be allowed! FAntastic, aweful and pitiable would generate three alphabetical openings!
The sentence structure needs to be maintained. Thus, a noun will need to start with an alphabet and its adjective would need to start with teh immediate alphabetic partner! Thus, Amazing Boy Catches Dogs Expertly could be the beginning of an Alphage! ( alphabetic passage)
If this gets too difficult, Try having each sentence beginning in alphabetic order.
Sometimes it would help to allow verbs like is or are irrespective of the alphabetic order.
Children show a remarkable degree of imagination and rep[ertoire....
Allow them to play with the alphabet. The only condition being that the order has to be maintained and the structure has to make grammatical sense! It could be an outlandishly funny flight of fancy and make no real sense in terms of real world. In fact, the less sense it makes, the more children enjoy this exercise. So you could have ghosts jumping from Canberra to Helsinkie. Or you could have people breathing in water instead of air and radiating fire... So long as flow is maintained, order is kept and the grammar is adhered to, everything is allowed.
So go ahead... enjoy this as much as I did in my Book Club!!!!