Azure on Microsoft Learn is set up to not only provide instruction but provide hands on labs which are free of charge. This is a very useful training tool:
https://learn.microsoft.com/en-us/learn/azure/
https://learn.microsoft.com/en-us/learn/support/faq?pivots=general
The Azure free account is available to all customers who have never tried or paid for Azure before (see the free account FAQ). While this means that we don't support creating a free account in a paid subscription, it wouldn't prevent people from creating separate accounts on their own (although I know you said you didn't want them using their own cards).
Depending on the type of training, Azure DevTest Labs provides a way for you to set a spending threshold for the self service VM use in your lab. This can be useful in creating development and test environments as well as integrating with continuous integration (CI) tools.