Action like this is pointless because this was an emergency situation and emergency legislation was passed. And if they wanted to be taken seriously, they shouldn't have gone down the Daily Mail sad face route.
Quite honestly though, if you want to take action against the government it should be for their total mismanagement of the whole pandemic. If they had acted effectively early on, as many other countries did when they saw what was happening in Italy and Spain, then we would probably now be in the position that Austria, Germany, Norway etc are of being able to safely loosen restrictions. The UK government acted too little, too late, allowing the virus to take hold in the community so that containment measures would not be effective not that they attempted to put many measures in place.
They now want to replicate the re-opening that's happening in other countries who have a much better grip on the virus than here because they handled it better earlier on. In trying to make economic gains over countries that went into earlier lockdown with their ridiculous herd immunity policy, the government has ended up with a situation that is probably far more economically damaging than if they'd just done a short, hard lockdown in the first place.
If there is successful legal action at any point, it will be from the families of health workers and carers who died because of policy failures, not some Mums who are complaining that schools are closed. For a small minority of kids it will have been genuinely shit, yes, and in schools we've been working hard to identify who those kids might be and get them back in. But these Mums are not fighting on behalf of genuinely vulnerable kids. Most kids, as long as they are safe, loved and well-cared for are extremely adaptable, far more so than many adults.