I'm not sure what you mean by half curly and half straight. My granddaughter's hair is curly. At that age, her hair was all close to one length but the weight of the hair pulls the top part straighter. Her hair has always been very curly and so it's never been stick straight but if your daughter's hair is a softer curl then the weight could make the top half stick straight.
I also don't understand cutting the straight hair until it almost falls into the curly. Why not leave it all about the same length. When curly hair is professionally cut they taper the ends so all the hair isn't the exact same length. Otherwise it's too heavy.
Are you saying that as her hair grows in it's growing in straight? or literally the hair growing out of the top of her head is straight and the rest is growing in curly?
Anyway, putting in product everyday isn't practical and could back fire when she's older and wants her hair to be perfect.
If the style doesn't look good natural, I suggest getting a different style. Also, if it's long enough, putting in barrettes might help. If the top is straight because of the weight of the long hair, a shorter cut might help.