My 4yo son received a 1099-DIV, do I need to include that in my tax reporting?

If your son is a qualified dependent on your tax return, then yes, assuming it's your dependent's SSN on the 1099-DIV. Under the Personal Income tab scroll down to the "Less Common Income" section and elect to start/update "Child's Income".

If the income was less than $1100 then it does not need to be reported on any tax return.  If the income is over 1100 but under 11,000 then you can elect to include that income on your tax return.  Alternatively you can file a tax return for the child for that income.  

Here is a link with more information on the making the election.  

In TurboTax under the "Wages & Income" tab scroll down to "less common income" and "show more."  There you will find the "Child's income" topic.  Links in that interview will also provide more information.