Our son is marrying this wonderful gal with a wonderful mother. The problem is they don’t have means for a big wedding. We don’t want to insult her mother, but we want to offer to handle it. We don’t want to burden the kids.
Ask to speak with her mother. Be honest about your concerns. Tell her exactly what you say here, that you don't want to insult her. She just may surprise you and graciously accept your offer.