Hat sizing

Can anyone help please ive found this lovely pattern for a ribbed hat but it only goes up to 9-12 months I wanted to make it for a 3 year old. sizes start from 0-3 (co75) 3-6 (81) 6-12 (81) 9-12 months (87). In cm starting for 0-3 months 35.5cm 3-6 38cm 6-9 38cm 9-12 40.5cm if this helps. Im useless at maths so stumped if anyone can help the pattern is king cole 3905

The ribbed pattern is going to make the hat pretty stretchy so you may not have to be so exact. It looks like the gauge is about 21-22sts/10cm. A 3-year-old’s head is about 19-20 inches (48-50cm). That gives you a cast on of about 99-105sts. Since the other sizes increase in increments of 6sts, this range is rounded so it’s consistent.
You’ll have to give it a try and see if it fits correctly. There’s a bit of approximation on my part involved.

Thank you