BMI/ centiles are a decent ballpark guide but not a complete picture.
Then consider body shape. Ribs should be visible not totally covered over. Prepubescent children should have straight body shapes. There shouldn't be obvious surplus fat at the belly, hips or chest- children are different shapes to adults.
What are their genes like? Are they following the way their parents and other family members have grown? Are they following their own growth patterns? When have family tended to hit puberty?
What is their diet like? Do they end up aquiring additional sweet treats from here there and everywhere? What do they eat/ drink when out and about? It's quite easy to have a decent diet at home but end up being tipped into a surplus by additional extras that find their way into a child's life.
What are their activity levels like? Do they have a lot of incidental exercise? Are they generally active or naturally slow and ploddy? How much movement do they actually get at their sporting activities? What's their functional movement like? Can they sprint? Have they got good stamina? Can they get up and down easily and move in a variety of motions? Can they climb easily? Have they got strength relative to their build? (I know some secondary age children that by late primary age were struggling to move functionally)
Some children concertina as they grow. My two tend to change face shape but there's little obvious difference in the body. Some children are more obvious. The awkward part of assuming they'll grow into their mass if their diet is continuously unbalanced for their needs is that you don't know when they'll stop growing and then the surpluses will accumulate. I've known heavy y7s stretch out through y8-9 then get heavy again once they stop growing- plus their appetites increased while growing rapidly.
Does being overweight matter? It's not a moral failing or weakness. Long term it is a significant trigger (or correlation) with poor health. That generally doesn't show up until middle age, but excessive fat (especially visceral fat) does stress the body's systems, and having that stress through youth is very far from ideal. Youth is when people's template for themselves is set- "fat" youths expect themselves to be "fat" adults. Their habits and activity levels are set then, and while they can be changed, that is very difficult to do and sustain. It's very difficult to reset to healthy weights after sustained periods of being extremely overweight. It's also just harder to move and less comfortable to be active when there is surplus weight.
Making sure children are in good health and making modest tweaks if needed is much easier than letting problems escalate into the late teen years and adulthood.