Our minor child was diagnosed autistic in 2024, should we check was disabled on the dependent questions?

Yes.  You can indicate disabled for the child but if they are under the age of 19, there is no "extra" tax benefit (for being disabled). 

If  the child is over the age of 12 and you have dependent care, expenses so you can work, those can be claimed on your tax return.

For tax purposes, disabled means unable to perform gainful work, due to a condition that is permanent or will last at least one year or will lead to death.  Gainful work means being able to work a regular job for at least minimum wage.  Because the tax focus is on the ability to work, many people with medically recognized disabilities are not disabled for tax purposes.  

I'm going to expand on my previous answer.

"Disabled" has very specific meaning when it comes to taxes.  It is not enough to have a medical diagnosis of illness or disability.

For most purposes, "disabled" means unable to perform gainful work.  Gainful work means work for money or profit, and the ability to work a part-time minimum wage job (or more) is considered by the IRS to be proof that a person can perform gainful work.  As a result of the focus on work, many people with recognized medical disabilities (vision or hearing impaired, amputee or wheelchair user, etc.) but who can still work, are not disabled for tax purposes.

https://www.irs.gov/publications/p524

https://www.irs.gov/publications/p501

Specifically to claim the child and dependent care credit, you can claim this credit if you need to pay for care for your child up to the age of 13.  At 13 and over, you can only claim the credit if the child is disabled.  For this credit, disabled means "unable to care for themself" -- meaning they can't feed, dress or clean themselves without help, or they need constant attention to prevent them from being a danger to themselves or others.

https://www.irs.gov/publications/p503#en_US_2023_publink1000203267

So the fact is that, just being diagnosed with autism may not be enough for your child to qualify as "disabled" for IRS purposes, either now or when they are an adult.  It depends on the severity.  Are they unable to clean, feed and dress themselves without assistance?  Would they be unable to work a minimum wage job (or better) when they get older?   It depends on the medical situation with each individual person.