The story itself only calls for six characters: Chicken Licken, Henny Penny, Loosey Goosey, Ducky Lucky, Turkey Lurkey and Foxy Loxy. (I decided not to use Cocky Locky because I had something else in mind for Mr. Rooster.) In order to get a performance out of young children, they really need to have their characters brushed with broad strokes. That is to say that any drama or comedy has to be easily understood and modeled from some performance they would have acquaintance with. It helps to be able to relate their actions, vocal inflections, and attitudes to characters they already know or are familiar with.
Then you can help them dig deeper to bring out even more through their own gifts. I decided to give each of these main characters alter egos from classic movies and TV characters - Lucy (Lucille Ball known for her physical comedy), Mae West (wise-cracking blonde bomb shell), Maverick (Western movie character), the Scarlet Pimpernel (English literary character), Sam Spade (savvy detective) and the Keystone Cops (hilarious physical comedians). Many of the mannerisms of these characters are much imitated in cartoons, and thus familiar when modeled for the children. The children would not know these characters by name, but the director should and thus have a starting point upon which to base the coaching.
The other difficulty with children's theater is creating a flexible cast with many parts that could fit either gender. Often, the girls outnumber the boys by a ratio of at least 2 to 1. But occasionally, the boys will not only outnumber the girls, but outshine them, too, in talent and ability. Since you never know exactly what the make-up of the cast will be for any given show, it is best to have a certain portion of characters that could be played by any age or either gender. So, I decided that the population of this cast would consist of the common fowl found in the typical English barnyard - chickens, ducks and geese with their chicks, ducklings and goslings. By using families of parents and children, that sets up familiar situation comedy to tap into.
In the youngest age group there will always be the children who want to be in a show because they love to sing and dance, but do not necessarily understand how frightening it can be to be up in front of an audience. Before actually casting a show for this population, I like to spend time doing acting games. Through observation of the childrens' improvisation, movement and interaction with each other I can usually tell which children will ultimately be too shy to actually perform and which ones are good candidates to actually come through with a performance. Then I can cast with confidence the main characters and put others into groups where they can feel more secure.