DD was born on the 91st centile and dropped to the 9th by the end of her first year.
She was born with a heart defect, and had very poor weight gain for the first year. Just before her surgery (at 13 months) she was painfully thin. You could count every rib down her back, and she had a heaving sunken chest. It was horrible to see.
Now, at 2.5, she is a happy, healthy, but very short toddler (still 9th centile). Her size may have been affected by her 'poor start' or she may be just destined to be short (SIL is 4'10") as there are 'short genes' on both sides of the family.
I am not the slightest bit worried about her. She has bags of energy - can run, jump as well as any toddler her age. She can climb as well as many taller children. She has good skin and a healthy mop of hair. Having seen her really ill, I KNOW she is absolutely fine. While she is not fat, she could never be described ass skinny now.
TBH, I didn't bother taking her to her 2 year assessment with the HV. She attends day nursery when I am at work, and they say she is meeting all her developmental milestones. Her cardiologist is not concerned about her height or weight.
As has already been said, there are many indicators of a child's health other than height and weight. The 9th centile is still within 'normal' range. Toddlers growth is not linear and can occur in bursts followed by periods of little or no growth at all. We measured DD at 2 and she was 83 cm. Two weeks ago we measured her again and she was 85.
There are advantages to being short as a toddler. DS had the opposite problem - he was really tall for his age. At two and a half he would have amazing tantrums out in public. As he was the height of an average 4 year-old, he obviously appeared very badly behaved. Now people think that DD is really advanced to be using the toilet when so small!