I agree OP. Children have been thrown under the bus by the government.
But it's not for individual shops to make up for that. As others have pointed out shops (and public transport, for the poster who mentioned that) have limited capacity and allowing children along presumably doesn't increase the spend per adult enough to cancel out the increased queuing time for others when hours are limited and people might go elsewhere.
You might well find that after a couple of weeks, once the initial rush is over, policies are relaxed a little. You might also find that some shops decide they've made the wrong decision economically and reverse their policy.
But it is still not for individual businesses to compensate for the government. That's not to deny how hard this is on children (and that's not to say it isn't also hard on those shielding).
Can't use our free coffees and DD was looking forward to some cake!
They should consider doing a family day or family hours - I will suggest that. Although they haven't replied to my request for them to extend some vouchers from April yet so I am not hopeful of a reply!
Free coffees is probably not top of any business agenda right now, no.
And as for a family day, if there is enough interest and it can be done with toilets, cafes and playgrounds closed then why not. I imagine the garden centre would need to charge an entry fee, redeemable against purchases, but as long as you're happy to pay that they might well go for it.