My husband got away with saying f instead of th all his life....when I met him I tried to correct it but he was having none of it. It is bad parenting. I made doubly sure my my two kids pronounced their th, and they still do today. But my daughter doesn’t think it matters and allows my two grandchildren get away with it. Of course my husband is in agreement with her, but I still correct them. My daughter should know better as when her daughter was born she really wanted to call her ,Thea, until I pointed out that to her grandfather that would always come out as Fear. She realised then that pronunciation is important. She tried to get her dad to say Thea in a last ditch attempt... but he couldn’t , or knowing her dad, he wouldn’t. So the lovely name got ditched , in favour of the one she now has, which I dislike intensely...but of course would never say it or show it!