As far as I’m aware it’s perfectly ok to change your Ts&Cs but the question is more how to go about it. You could email out to all clients that due to the current climate it’s been necessary for you to update your contract, send a copy and either ask them to sign or at least give ‘due notice’ that the new conditions will begin one month from now. Invite the parents to contact you to discuss any concerns about the changes within 2 weeks, so they have the option of opting out if they are unhappy. If they opt out you at least know the slots are free for other potential clients. That also gives you scope to make individual arrangements on an ad hoc basis. If you do this, start each email thereafter with a reminder that new conditions will come into play on “date” and these will supercede previous arrangements, again reminding parents to contact you to discuss.
Otherwise, you could speak with parents individually and ask them to sign a new contract with you.
It would be interesting to hear whether anyone else has done this. I imagine the transition from pre-pandemic to pandemic therapy may have!