It is best to use pure music, which may also be helpful for children's musical literacy in the future. Let me recommend some songs to you.

Piano Music My Soul (Like "Happy or Sad" in Baidu) Canon

The soundtrack of Joe Hisaishi Miyazaki Hayao's movie "The Truth that You Leave Chihiro's Summer in the Sky".

Autumn language in Richard Clayderman, dream wedding, Adeline by the water.

Anne Wonderland in Bandari, The First Snow, etc.

The level is limited, that's all I know.