Hi op, opinion seems to be split on this one. Several posters further up-thread made a good suggestion that you phone your dm (better than texting/e-mailing, perhaps).
There were some fair, non-confrontational suggestions (also up-thread) of how you might word an e-mail/text message, but you could print these out and use them as a prompt when you speak to her.
Only you know the exact details of what was agreed with your dm. On the face of it, based on your posts, you were not expecting your dm to ask to be reimbursed, so this could be the starting point of a frank conversation about money/expectations (which would then set the tone going forward). Depending on the outcome of the conversation, you could offer to pay for her train fair, which may have been expensive (in instalments, given your financial situation). It seems to have been your dm's choice to take the children out, even when there was food provided at home, so perhaps she should not be reimbursed for that. Should your dm mention the 'thank you' dinner, you could then mention that it was paid for in vouchers.
If this is a one-off it's possible to salvage the situation (even though it's clear to see why you are upset). If this is an example of a pattern of behaviour you may like to post on the Stately Homes thread, where you will find other posters who have had similar experiences.