Very often, I hear parents tell their toddlers or young children, "What do you say?" after receiving a gift or help from someone. I have one thing to say, "Don't do it."
Most often, toddlers and children don't respond to this well-intended command for one simple reason - they aren't ready yet. One thing I know from both teaching and parenting is that an obsessive amount of modeling and mimicking leads to developmental and learned changes, whereas a well-intended command often leads to a blank face.
The better thing to do with our toddlers and young children is to use, "Say thank you," with a slightly high pitched tone at the end to note an invitation, rather than a demand to perform. Instead of a blank face, I believe toddlers and young kids respond differently to this type of modeling and mimicking. The truth is that toddlers' and young kids' cognitive brains are still developing, and though we might think my child knows what to say, the reality is that they might NOT actually know what to say in the moment without copying and repeating you.
Over time, with enough modeling and mimicking, they will begin to say "Thank you" on their own, without being asked to do so. This is not just a good way to teach them how to say "Thank you", but a way to help them do lots of things in their developmental growth and learning.
*Photo used with permission by Alexander Lyubavin at https://www.flickr.com/photos/santea/