As a general rule, parents must encourage and facilitate children having as much contact with both parents. This may not always be ideal and that is why all decisions regarding custody and access are made on one standard only – the best interest of the children! NOT the parents’ wishes.
The ability of a parent to care for a child is a consideration in determining the best interest of a child but absent any special dietary concerns, feeding the children candy and junk food in and of itself is not likely to be considered reasonable grounds for witholding access.
My advise is – talk to your ex about this in a non-confrontational way. Maybe have a respected 3rd party such as a mediator help you with it to ensure that your message is communicated properly and that the other party doesn’t feel judged or that you are controlling him or her.