Was your Tamagotchi already the cheerful family (Mememamatchi, Memepapatchi and "cheerful" children)?
If so, they should start off as the cheerful family but they will return to the blended family if they evolve into "non-cheerful" teens (Mamekatchi, Chamametchi, Sherutchi/Shelltchi and/or Korokotchi).
To prevent this happening, always use the second item for training.