No matter what your kids age, have your kids practice their writing by starting a journal. They can practice letters, numbers or write about things they are doing over the summer. All you need is a simple notebook or some paper that you can just staple together.
They can also add pictures or scribbles if they are toddlers.
Adventures in journaling shares some free printables if you think your kids will like a fill in the blank type. Sections include items such as to do lists, memories, recent things learned, and more. The site offers both color and black and white versions.