Personally I don't think telling parents who are probably already anxious and feeling guilty that lockdown will damage their children irrevocably is very helpful
But you absolutely need to be providing them with the information required to limit the harm, this is the biggest failing of the CMO entirely, completely ignoring any information or recommendations on how to limit the harm of lockdown.
For example, we know daylight time outdoors is a huge protector of myopia, so all children should be spending many hours a week outdoors in daylight to reduce the risk of needing glasses more than 50%.
We know the risk of avoiding diabetes and obesity is correlated with exercise and all children need to be getting the required hour of aerobic activity and the required strengthening activities, replacing that lost via school, extra curricular activities and playing with friends.
Not doing those things will harm your children, the medical evidence is very secure, but the CMO has ignored it, never mentioned it, schools might not so essential if he actually updated his guidance on healthy living.