Just message her, thank her for letting you know that her mother will be in town for 4 weeks and taking the children and remind her that because she has already sent her schedule and your plans have been made around her needs, that payment will still be required. Finish up with an statement detailing the date you are expecting the children to return to your care or asking for a firm date of when they will return to your care and the payment schedule reminder for this time.

That then clearly lays out your expectation of payment for her change of situation and see how she reacts. Do it in writing. E-mail, text, FB message - but do it in writing.