Pick a short phrase your child likes — 'here we go' works well.
Say it in a giant voice, then in a tiny mouse voice, then a slow deep voice. Exaggerate hard; the contrast is the point.
Invite them to copy each one. Then let them pick the voice and you copy. End on the tiny voice — it settles the room instead of winding it up.
Playing with volume, pitch and speed builds prosody — the musical layer of speech that carries meaning and emotion. Children who control prosody are easier to understand long before their articulation is fully accurate.