Hello ujwalmk07,
Welcome to Apple Support Communities.
We understand that your children are not able to use Apple Music on their iPad? Are they using the same Apple ID for Apple Music that they use with Apple One. If not, we recommend that they use that Apple ID? if they try to sign in to Apple Music using their Apple ID what happens? What is the exact error message, if any? Are they able to listen to Apple Music on their other Apple devices, such as their iPhone or Mac? These answers will help us to isolate the cause.
If you have completed these troubleshooting steps, we recommend that you contact Apple Support here: Apple Support
Let us know if you have further questions.
Take care.