Often when a plural is possessive, children become confused as to where the apostrophe should be placed. A game they can play to practice involves a number of sentences along the lines of: "There is one shark. Its teeth are sharp." Below it, the children should write this as one sentence, with a possessive noun and a common noun. In this example, the answer would be: "The shark's teeth are sharp." The apostrophe comes after the "k" because it is singular. If you then introduce plurals, like "two bears," "three snakes" or "four sheep," the children will have to think about the answer more.
This game involves the use of magnets. These are readily available from gadget shops and are usually not too expensive. Use tape to attach a bit of paper to each, and then write a word on each magnet. They should all be possible plural versions of the same singular word. For example, the plural of "class" could be "classez," "clases" or "classes." If you then make a fishing rod with a stick, a bit of string, and a magnet on the end, the children can fish for the right answer.
On a sheet of paper, write a list of singular nouns in one column. In a second column, write their plural forms. Use scissors to cut them out, then mix them up on the floor. The child playing the game should then rearrange the words into the two columns, with the singular words all in a line and the plural words all in a line.
Instead of working solely with words, introduce pictures to help stimulate the brain. Draw common objects on a series of cards and show them to the child. Ask the child to then write down the plural of that object. As they become more confident, introduce words ending in "y" like "lolly" or words ending in "s" like "bus."