Hello,
when creating child products in VM 3 they are created as normal products, and thus also listed in the sitemap.
As i guessed the problem is not related to child products itself.
The problem is that child products are orphan for category, they are not assigned to any virtuemart category.
When generating the sitemap where products are grouped by category, the product needs to be assigned to a category of course.
if products are orphan of a category can't be included and categorized in the sitemap.
If you can't assign a category to all products a alternative solution is to create a new data source that lists all products without category grouping, so that all products will be included in a plain list without categorization.