Well done OP for acknowledging it, it can't be easy.
My DCs are older and I've seen the cute, chubby toddlery yRs go through primary school, stretching, growing, and filling out, and by y6 they are visibly carrying surplus fat and it gets much harder to address as they gain more independence. One child I know at the upper end of primary, struggles to do anything physical now, can't do climbing walls, assault courses, walks very very slowly to the extent that on an organised activity, the check points were making emergency phonecalls to find out what happened to the group. It's sad that the child is facing such barriers to living healthily and becomes more and more sedentary. They'll have a big issue in learning healthy habits later in life.
So great to start now, and it doesn't take much to change. Eat a rainbow is simple advice which means that you'll get a good range of nutrition. I cook breakfasts which sounds a lot, but heating some beans or scrambling an egg doesn't take longer than the toast, but gives a greater range of nutrients and fills for far longer than cereal. Plus the price of wheat based cereal is now going up sharply.
Something that shocked me when I realised is that those children's bowls from IKEA can take a whole tin of soup/ beans! We had other melamine children's bowls that were at least as big as the bowls from a 1980s dinner set. It's very easy to think you're giving a sensible children's portion when it's what an adult would have eaten 40+ years ago.
Live actively; clubs/ sports are great, but just getting out and moving is important. My oldest is now at secondary and beyond walking distance, so I park the car further away for him to walk to me. It's bonus exercise and helps him build up for the walk to the bus stop and keeps walking as a habit.
If there's one nearby, junior parkrun is for 4+ and lots of families participate together, including prams. Normally 4yo pace starts as a brisk walk for an adult and many families come to improve their fitness together. Even starting with one lap to get used to it is a great way to start Sundays.
We're coming to the end of winter and it's a great time to start refreshing your diet and getting more active as a family.