Anyone who knows about weight gain in children?

[Schooltripping]

Dd (12) has a small appetite and is slim for her age. When she was weighed at the end of Year 6, so approx 12m ago her height was 138cm and weight 25.6kg. She is now 145cm and weighs 26.9....does this seem like reasonable weight gain for her growth, or how do you know when to be concerned?

On the NHS BMI calculator she is below the 1st centile both times so that doesn't say anything about her growth in that time. www.nhs.uk/live-well/healthy-weight/bmi-calculator/

On this chart it doesn't look like she is gaining enough to track along the same centile line.
www.rcpch.ac.uk/sites/default/files/Girls_2-18_years_growth_chart.pdf

Her growth in height looks fine, but I would seek a medical opinion on her weight to check everything is okay.

Thanks so much for the links, that's really helpful.