Third world countries are going to become first world countries, barring as disaster. When they do, they'll use a lot more resources per capita, but their population growth will drop dramatically. Switching them over sooner is a boon for the environment.

Keeping people poor or would be a soft racism/proto-eugenicism, but reduce child mortality and empower women with contraception (as an option they can choose, not pushed coercive) are pretty unambiguously good. I mostly donate to the Against Malaria Foundation both as my anti-poverty charity and my pro-environment charity. They're rated well on effectiveness.

I haven't done much research into highly-effective charities that provide contraception or related educational materials, but https://www.thelifeyoucansave.org/causes/women-charities has some that seem relevant. Women's rights in general is usually a pretty good proxy for both "let women not have babies when they don't want to" and "give women options for fulfilling lives other than just having babies".