We'll be 2 adults, 2 children in that room now, rather than the 1 adult, two children on our current booking for that room. We'll definitely let them know either way but very interested to know if there will be an additional charge so we can prepare financially for the charge.
You should be fine with no "additional adult" charge. The base room rate includes up to two adults per room. The additional adult charge is for the 3rd and 4th adult. So 1 Adult + 3 Children is the same rate as 2 Adults + 2 Children.