During a nappy change, say what you are doing as you do it: 'now the left leg... now we lift... all clean.'
Slow down slightly and exaggerate your intonation. Look at their face between sentences rather than talking over the top of them.
When they make any sound at all, stop and respond as if they had spoken, then carry on.
Nappy changes happen several times a day, every day, which makes them the highest-frequency language slot available. Narrating turns a silent routine into hundreds of repetitions of words attached to the exact events they describe.