I think you might need to look at content marketing strategies for this. Maybe design some landing pages and direct traffic to it. But you could also think about how this website shows it's child websites, stack exchange has it's global nav and the child websites sit inside of them.
Depending how much you want to highlight the relationship you could use one of several approaches:
Do like stack exchange: add a link to the child site on the parent and vice-versa.
On the parent site: Add a section "built with our platform / facilitated by us / our events" and showcase / link to some / all the events, with a link to the child site. Or just ...