I recommend you decline. As former Catholics, your daughter and her fiance almost certainly still are bound to observe the Catholic form of marriage, and their marriage in the non-Catholic church will be invalid. While there is no official Church teaching on this issue, lighting the unity candle with your daughter would send the wrong message: 1) tacit approval of a marriage not recognized by the Church, and 2) great potential to cause scandal.
The Catechism of the Catholic Church defines scandal as".. .an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor’s tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense" (CCC 2284).