It depends on the age.
If they were like 4 or 6 or 8, you know a little kid, then I would help them lose weight. Make the groceries healthy, encourage them to play outside for a few hours everyday, put them in some sports and recreational teams and such.
But if they were like 12 or 14, like a teen / older kid, then I would let them be overweight honestly. I think it would be very detrimental and hurtful to tell someone who's going through puberty with low self-esteem to lose weight, and make them feel bad about their body. That's how kids grow up and end up hating their parents / saying their parents always made them feel horrible about themselves. It'll be completely their option to keep that lifestyle in a few years by the time they're adults.