In most places before a child is able to start Head Start the child must be at least partially potty trained and out of diapers.
The child starting head start may go to head start when they are still potty training but they must wear pull ups so that they can use the toilet in head start and not regular diapers.
So if your child is beginning potty training and hasn't been fully potty trained yet then yes they can start head start but they must wear pull ups to head start until they are fully potty trained.
It's best to have them potty trained fully if possible before head start and if not I would just wait and send the child to kindergarten instead once they are fully potty trained.
But if you want to send your child to head start then that's okay too even when they are not fully potty trained but they can't go to head start wearing regular diapers and not use the toilet at all.
The head start teachers will not change regular diapers but will help change pull ups but they are not there to change and wipe your kids butt when they can't use the toilet fully.
Now if the child is special needs and going to head start and needs to wear regular diapers full time then that is different and they will make arrangements to allow your special needs child into head start with diapers.