If you are actually aware that she is allowing others to use her kids social security numbers and then filing tax returns, please report it to the IRS (or some other legal agency, but thec
IRS is probably the one most likely to actually investigate it). If it doesn't get caught early, the identity theft will ruin her children's lives. Even obvious identity theft costs thousands to clear up and the children will have to turn in their mom to clear it up anyhow. It's better to do it now and minimize the damage.