Gonna treat the little one to a bike for his birthday but not sure which size to get him. He’s about average size in his class at school.
Options are
Frog 52 (too small ?) or 55 think the 62 will be to big as it says age 8-10
Islabikes beinn 20 large or beinn 24 (too big?)
Anyone that’s bought one advice would be much appreciated

I would say a Beinn 24, but check the Islabikes size chart against his measurements (they’re good figures, not a rough guide).

http://www.islabikes.co.uk/bike_pages/pdfs/sizes/Size_Chart12AW_web-c2.pdf

My eldest (8 1/2) has been on a small Beinn 20 since he was 5 and realistically it’s been a bit small for him for the last year (although he’s now got a great low bar position on it!). I’ve certainly seen lots of kids smaller than him happy on a Beinn 24.

Plenty info on the islabikes website re sizes. Try measuring him, that’d help you more than asking what bike for a seven year old that is of average height in his class.

Our youngest turned 7 in July and got a Beinn 24 for his Birthday. he is one of the tallest in his class. Now 132cm and bike fits him great, with a bit of seatpost up

Was in Islabike the other day and they looked at my lad (6.5) and said Large Beinn 20. If you are local to Ludlow they do a free fitting service and have a test track out the back of the factory to try before you buy :O)
Nice people.

I’ve just bought my 7 year old daughter a Frog 62 having outgrown a Beinn 20 (older version).
There is less info on the Frog site re sizing (compared to Islabike site) but to give you an idea. Daughter is 1.28m high, inside leg about 57cm but standover height fine – about 3in of seatpost showing. Reach on the Frog is IMHO quite long – it’s about 19.5in c-c top tube. Although she is managing OK, I’ve just fitted a 40mm stem (instead of 90mm). Hoping it will last her for a while!