Creative writing online is the best medium to avail resources to find good and quality exercises. There are numerous websites that do not require any subscription and avail free writing exercises like poetry writing, fiction writing and more for all age groups.
Most of the websites offering its online practices and exercises have a provision for self tests by which a writer can test his / her writing abilities and skills and improvise. A lot of other websites offer creative prompts, which can be used to begin writing a story. Websites that offer simple classes and training for developing language proficiency and writing techniques are also available.
Another place to hunt for good creative writing exercises is a well established library or bookstore. Before Internet, lecturers and writers would go through a lot of old and new books in the market that included writing exercises for children and other ages. These Master papers service are now also available online at much lesser costs.