Sizing for kids

Could someone please give me an idea of when to upgrade the bikes for my 2 kids. My son is currently riding a 20 inch wheel and is 130cm tall, and my daughter is currently riding a 24 inch wheel and is 150cm tall. I think that they're nearing the point of going up a size but I read a guide online this week saying they're perfect on the sizes they're on.
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Is their riding position correct in terms of saddle to pedal height and the reach to the bars giving the right riding position, if the answer is yes then the bike fits whether it runs 20" wheels or fit 12" wheels in the same frame!