Glucose is osmotically active, this isn't relevant with lower concentrations and when cells can take said glucose in, but it is in DKA where insulinopenia makes glucose a much more effective osmotic agent.

Brain cells create endogenous osmoles in hyperglycemia so that water isn't sucked out of them. If we lower glycemia too quickly those same osmoles endup causing water to go IN the cells: cerebral edema.

Also, using bicarb even in adults is controversial. There might be benefit in severe acidosis that causing hemodynamic compromise. In DKA, giving insulin and dextrose causes metabolism of ketone bodies and creation of bicarb anyway, going for the an aetiological treatment is much more beneficial an less prone to unwanted side effects.