The quick answer is: no you can't. A quick Google shows that there are some workarounds that may work, but all of them require either a separate Mac that is also available, some dodgy software that may include malware or a lot of tinkering for something that might work on a good day. iMessages and Facetime are cornerstones of the Apple ecosystem and the company is just not interested in allowing Windows users acces to it. Your best bet is to train the grandchilderen I'm afraid.