Hi OP,
some interesting and good advice on here. I'd just like to add one perspective: Your children are still very young (your eldest would be in their first year of schooling in my home country - I started school at age 7.3 and was able to read a little (think ~yellow band) and was thought of as extremely advanced). And I think up to a certain age, the learning that happens through the immediate experience of the world around you is more crucial than abstract, mediated things like reading, writing and maths.
I am also convinced that nearly every infants-aged child has the potential to learn more, academically, at an earlier age, than they do at school; as in, children are mentally capable of learning more, faster. School is not actually very efficient at teaching these things.
Parents who support their children at home by teaching/practising stuff in addition to sending them to school tap into that potential.
However, just because children are capable of learning academic stuff at an early age, does not mean that they should, that it is the best thing for them in the long term. IMO for young children immediate, direct experiences are more important than abstract, mediated learning, even if they are capable of the latter.
Of course if you have the time, you can do both. As many home educators indeed do. But if your English Infants aged kids are at school full time, then the one thing they are most likely to miss out on is direct, immediate experiences of the world.
So if you have a little time to spend with them, you can choose: Give them time and opportunity for direct, immediate experiences, or tap that extra abstract learning potential. Up to age 7 or 8 I personally would always go for the immediate, the concrete. I would choose half an hour of cycling around the neighbourhood, experiencing distance and direction, topography and landscape, balance and speed, over studying the geography of Kenya. Using bricks to build a castle, experiencing how amounts and shapes work together, over practising number bonds. And even building a den, quarrelling with siblings, doing chores, going along for shopping, hanging around feeling 'bored', ... (gaining the experiences that will enable/strengthen reading comprehension) over reading about fictional children building a den, quarrelling with siblings, doing chores, going along for shopping, or hanging around feeling bored.
So, perhaps OP your children ARE 'behind' - behind where they could be if you worked on their academics with them, behind their peers who do extra work outside of school. But perhaps you are doing them a favour!