How can I promote a child website?

We have 2 websites - let's call them website A and B.

Website A is about event management and website B is about sustainable events, so they are very similar, albeit with slightly different themes/goals.

In essence, website B is the child of website A.

How can I show on website B that it is powered by website A?

How can I promote website A through website B without having to put in obvious ads/banners/pop-ups?