Add the data annotation "order" to the property of your classes (see below)
i did test it and it does work, alas not all of the data annotations are working if you read this forum a little more.
but again thankfully order does work.
Indeed setting the order attribute is the only way of controlling the order of appearance of data fields in RadDataForm so far .
We are open to any improvement suggestions and in case you have some ideas on other ways it can be controlled, do not hesitate to make a feature request.