When to monetise - education/learning site

Plan is to use paywall/subscription model - just trying to understand when to pull the trigger because I think it will affect my rankings due to lower engagement rate. Also, once I start charging, I think it will limit natural backlink growth.

There is a small opportunity to gain affiliate income for books etc.

I have trouble understanding how ads would be a suitable monetisation method, considering it’s mainly children on the site. Maybe I’m wrong but why would they be clicking on the ads?

I’m targeting 1000 clicks a month as the start point for subscriptions. Currently at approx 300 clicks P/M, expecting to reach 1000 by September.

Thanks for your input!