I have a 4 1/2 year old with tight "S" shaped curls.
Right before washing I wet her hair then put some adult conditioner in it. Like Pantene or Suave for moisturizing. Then I wash it, rinse it, condition again. Then rinse but don't take all the conditioner out.
I braid it in the night & in the morning to comb her hair I loosen the braid, spray it lightly with a water use wide tooth comb to untangle any tangled hair & then brush it.
Sometimes I just comb it & re braid it for the day or let it loose in a ponytail.
I find that the conditioner in her hair & re wetting it with the mist helps in combing her hair. Of course section off the hair. Hope you are able to figure out how to help your daughter w/ the tangles.
I don't think diet has anything to do with tangles. As an adult my hair gets tangled not as much but it does.