My baby had an allergic reaction to a johnson moisturizer and she developed a rash on her face and neck. she now has 2 skin tones due to that since the rash dried up after changing back to dove. what can i use to retain an even tone?

Patience: If you stop applying things to the skin & just keep it clean it will eventually return to balanced tone. It could take weeks/months but it will. In the future try a small amount of such products before use. Apply it to the pad of a Bandaid & place it overnight in a specific site. If the area of the pad does not react after overnight application her skin can probably handle it.