We began using nature journals for science about 7 years ago. Don't be fooled however, nature journaling teaches them much more than science. They also learn observation, art and basic research, not to mention they understand that they are able to learn on their own. They realize that they can go out, see something they don't understand, and then find the answers. They learn to be their own teacher.
When our children are old enough to begin a nature journal, between 4 and 6 depending on the child, we allow them access to some good quality art supplies. We use Prismacolor pencils. They allow our children to add detail and color and are easy for the little ones to manipulate. We have given our children more options as their art skills have progressed. These Caran D'ache water color crayons are my new favorites. I will suggest that if you expect quality results give them some quality tools. (Trust me on this one, I don't recommend spending money very often but nice art supplies are worth the investment IMO.)
The first assignment that we give each child is for them to go out into the yard and look around very carefully. They are looking for something that they have not noticed before. It can be the veins in a leaf, the spots on a fuzzy caterpillar, a blue rock or the fact that our dog has her tongue hanging out the side of her mouth. When they notice something that is new to them, they get to draw it. They must look at it while they draw and we encourage them to try to notice even more details while they are working. If our child notices a bird or something else which would not stay put long enough to draw, they may look it up and draw from a picture in a book or online.
If your children are like mine, their first drawings will not be impressive. It is okay if the drawing is immature and even unrecognizable. Their art skills will improve over time. At this point just encourage them to notice details and to try to include them in the drawing. So if your child noticed the veins in a leaf, and has drawn an uneven circle with lines going who-knows-where, that is wonderful. They have learned to look more closely at nature and to communicate what they discovered through art.
Next we have the children write something about their drawing. For the example of the drawing of veins in a leaf, we would ask what she just drew. If she says, "I drew lines in the leaf," then we would direct her to a source where she could discover what those lines are called. If she already knows that they are called veins then we would ask her to find out why a leaf has veins. The idea is to require them to find out something new and write that beside their picture. As soon as she understands how and where to find information, she no longer needs help with what to write unless she is having difficulties.
We require one nature entry a week. When I look at their assignment for the week they are usually full of information that they want to share with me. After we have discussed their page, I will usually tell them what I want them to do for the next week. Often the assignment will be exactly the same as the one I outlined above. However, if I notice that a particular child is struggling to do meaningful research or is stuck in a rut, I will give them a more specific assignment to help them progress in their ability. For example, if they are failing to convey new and meaningful information on their nature page, I would give them a specific question to answer or a process to explain for their next assignment.
If you don't have time to do anything more than this, your children will still benefit greatly by taking some time out each week to observe, wonder, draw, and study God's creation. However, I highly recommend the Handbook of Nature Study. We use this to give us more focus and to help us go deeper into a topic. You can work your way through a section doing a page on all the plants (or whatever) that are native to your area. It is an amazing resource; the only nature text that you will ever need.
Allow your children to enjoy journaling. As something interests your child, let them explore it further. We have studied rocks, leaves, squirrel habits, butterfly life cycles, live bearing fish, and the reproductive process of our pet guinea pigs. Hmm, I should probably get my children to produce some nature pages about ants.
If your children tend to focus on different aspects of nature, set aside time for them to show each other their books and talk about what each one is learning. This expands everyone's horizons.
They don't have to learn it all in a week. They don't have to produce a work of art every time. It is the process that teaches. Relax and enjoy.