Technically, your ex-wife has no duty to support the children (unless she adopts them). Therefore, her earnings or income would not necessarily be relevant. The larger question,
really, is why does your ex-wife have custody of your daughter's kids. Sounds like you may want to re-visit some of these issues.
Please feel free to contact me, or have your daughter and son-in-law call me.
Mark S. Guralnick, Esq.