Lots of individual choices on here, and that's fine, we all make our choices. But human nature hasn't changed. It's our environment that has.
OP asked what we could do - here's my two cents: Tax high-sugar foods and drinks to the extent that they are not cheap enough to be appealing any more, so that they don't line the walls of every corner shop. Give tax breaks and subsidies to healthier options, so they do.
And make going to parks, pools, indoor play, cycling, scooting, ice rinks safe - how many of us let our DC just go out and play? With all the talk of grooming, stories of acquaintances who've had creepy things happen, etc etc, we don't do this anymore, for better or worse.
And if we work, have other DC, or for many reasons, we can't sit in parks for 4 hours a day while they are active, same for swimming pools, soft play, sport events -- we work, we make dinner, we do toddler bath/bedtime, we get degrees from OU, we improve our qualifications, we single parent our families, we juggle our shifts with our partner's shifts, we struggle to pay for our housing, we work long hours, or we struggle to work enough to pay private school fees, or any number of things. Councils could provide active, supervised, after-school activities, supervised adventure park/playground/sport at weekends, traffic-free cycling and scooting routes to these venues, organised outdoor treasure hunts, you name it. And that's just 2 minutes of my brainstorming.
All of this would cost money, but not as much money as the NHS will spend on obesity-related problems. And some could be funded with all those taxes from all that sugary junk food.