Hi there @Elizabeth! We have kiddos in similar grades! My oldest two are 4th and 6th this year. We love nature study and have been learning a lot about it over the past several years. A few resources that have helped us a lot are Exploring Nature with Children (this is very Charlotte Mason friendly) as well as The Handbook of Nature Study. Shining Dawn books (which appear to now be called NaturExplorers) also have thematic nature studies. Ambleside Online has a nature study schedule for users of their curriculum, but anyone can follow that schedule, and Simply Charlotte Mason has a few wonderful resources at that link as well!
As far as supplies, we keep a small plastic container along with nature notebooks (for sketching, taping in things we find, etc.), some good sketching pencils and kneadable erasers (which are great for removing sketch marks), and various sizes/types of magnifying glasses (including a jewelry loupe magnifier which is great because it’s tiny and has a great lens) are things we keep in the plastic container (notebooks are either carried out separately or more often left inside to sketch in after we’ve explored).
One other item we bought was a wooden adjustable drawer organizer like this one. It is great because we can bring in our treasures and drop them into slots (and even resize slots as needed) so we can keep them to look at through the week.
Nature study can take as long or as short a time as you need. If it’s something you’re new to, start with short walks, periods of observation, etc. The Handbook of Nature Study has a website with some outdoor hour challenges here that they suggest you start with. So that might also be a helpful place to look!
In our nature journals we do animal sketches (there are great online tutorials) as well as general outdoor sketches. We love using watercolor pencils and blender pens because it creates a great watercolor image without soaking the page or getting out messy paints. But my oldest always prefers to just sketch with pencil. One great tip I read once though was to always include the date and general weather on every sketch page so you can remember what it was like on the day you observed those things.
Feel free to ask any questions–I hope these resources will be helpful! Good luck!!