Early childhood students aren't usually very good at writing down what they know or presenting things to a class. The best way to make sure they've understood what you've taught is to ask them. Ask your class in simple language to recite back to you what they have learned during a lesson. If you notice that some students aren't answering, take a special interest in those students and find a time to talk to them individually about what they've learned.
Young students love to talk about what they know. Once you present them with a lesson or a concept, ask them to teach it back to you. Once they put a concept into their own words and tell it to you, they are not only showing you that they know the concept, but also that they are able to know it well enough to put it into their own words. This shows they've synthesized the information.
Play the "Remember When" game for early childhood students to talk about concepts you learned a few weeks or days ago. Begin by saying "Remember when we talked about ..." and then mention a concept or lesson you taught. See what students say. If they still remember the concept and can tell you about it, then you know that you are teaching them in a way that promotes long-term learning.
Have students share what they've learned with their parents or siblings. Send a note home to ask parents to talk to students about a certain subject and report back to you with an idea of how much information those students have retained. This way, you will get an idea of how much you are teaching students that they are taking with them outside of your classroom.