Right. This is really common. 9/10 won on appeal. IPSEA is useful. Assessment is necessary if two legal tests are met:
1) that the child *may* have special educational needs
2) that those needs *may* require a plan to be met
Without assessing, you can’t for sure that either aren’t met.
Check out the IPSEA website. Refer the parents to IAS. Ask for next steps meeting with the LA, often the SEND office can over ride the decision to refuse to assess to avoid tribunal. That will depend on your local authority.