Children gradually adopt and enact the behavior that is modeled around them. Expressing your own gratitude regularly will teach them, over time, to do this also. (A side benefit is that it will make you feel better right now!)
Take some time to think about the things your child does, says, and is that elicit gratitude in you—from how they snuggle to how they smell after a bath to the sound of their laughter to the ways they are growing bigger, stronger, and more independent. Then, make a point of expressing your gratitude to them, saying things such as,
"Thank you so much for that hug. I love your kindness and warmth."
Or, "I really appreciate that you helped clean up after dinner. You were so thorough wiping off the table."
Or simply, "I'm grateful that we got to spend so much time together today. I love your company."
See if this practice helps you notice and verbalize even more wonderful things about your kiddo than you were previously aware of.