Online homonym resources include lessons on YouTube, BrainPOP Educators, First School Years and Homonym Game. The age of the children and whether they can read affects what resources work best. The ideal resources depend on whether you are using a strict or loose definition of "homonyms."
The term "homonym" can refer strictly to words that are spelled and sound the same but have different meanings, or it can have a more open definition, referring to any two or more words that sound the same. Most of the resources described here teach about homonyms in a more loose sense, as two words that sound the same but have different meanings.
One way to find homonym learning resources is to type "homonym lessons for kids" into the Search Bar on YouTube. The results include at least three homonym songs by different authors as well as a homonym lesson by Mr. Palinidrome's Kids Vlog, as of February 2015. A more interactive option is the Homonym Game at the website of the same name, which combines rhyming, poems and images to help kids answer fill-in-the-blank questions about homonyms.
If you are looking for online lesson ideas, "Homonym Activities for Kids" at BrainPOP Educators includes ideas for activities in various settings, whether at home or in the classroom. There are also a few lessons and worksheets on First School Years, including a page of jokes making use of homonyms.