How to make the amount field read-only?

I have made a form (below) and just tried it out but if you enter your child's correct day and time and it gives you the amount.  You appear to be able to amend the fee manually to whatever you want and go through to the payment.

So for example I did a level 3 class which was £154 and then amended the amount to £150 and when I got to paypal/payment part it still said £150.

Should the 1st Child's Fee and 2nd Child's fee be protected.

Thanks

Yes, I also believe it should. So, my suggestion would be is to make those fields ( 1st Child's Fee and 2nd Child's Fee) read-only so they cannot be amended manually.

Here's how:

I hope this helps. If you have any further questions regarding this, please feel free to contact us again anytime.

Thank you.