You may already have these in your collection, but some good resources are:
Kind News has been a humane education staple for decades with two age-level versions (resonably priced) https://redrover.org/readers/kind-news-magazine/
Gryphon Press has lots of humane education titles with some Spanish language translations & bulk discounts for 501c3s (more expensive even with generous discount) https://www.thegryphonpress.com/
The Maddie's Tail Wag books are popular, also available in Spanish (free) https://www.maddiesfund.org/tail-wag-request.htm
This program seems comprehensive & well-established. Reasonably priced and offers a digital version. https://education.muttigrees.org/about/
Some creative ideas here. Partnered with Scooby Doo and offers toolkits and seasonal grant opportunities. https://www.generationon.org/projects-and-causes/animal-welfare
As it may be of interest, the YMCA is hosting "Healthy Kids Day" soon and asking for community partners. In GA, our events are 4/25, but may be different where you are as the "official" day is 4/18. https://www.ymca.net/healthy-kids-day
I'm just starting to put together a humane education program as a result of a request from our local high school. So, I can't be much help on content & curriculum. I can tell you that I've been disappointed in what kids don't know given the decades of advocacy, especially considering the demographics of the high school. It seems small, focused sessions or events are more productive as the kids have so many other activities competing for their time. That may be different for younger kids.