The sentence "Termites mix soil and their spit to build their tall homes." is mostly clear, but it can be improved a little bit for better grammar and clarity.
Here's a corrected version:
"Termites mix soil and their spit to build their tall homes."
This sentence is actually grammatically correct! However, for even clearer understanding, especially for young children, you can say:
"Termites mix soil with their spit to build their tall homes."
Adding "with" makes the sentence smoother and clearer.
So, the best version might be: "Termites mix soil with their spit to build their tall homes."