is shown on Weblio as 'to knock down'. In this case, it would read as 'Kids who have been knocked down' or 'Kids who have been affected'. What they have been knocked down by is the summer heat .
Some of the uses of are to say that something has 'appeared', 'been revealed', 'shown itself', or more relevant: 'been produced'.
While the translation that I would suggest would be 'Some children have been negatively affected by the summer heat, we're careful regarding what they eat.', I think other (more paraphrased) translations such as 'Some kids have become ill from the heat. One must be cautious with their diet.' would also be fine.