My parents in law greatly surprised me recently by telling me that they think my daughter has a serious speech impediment. That being that she can't pronounce s or f. I have noticed that she doesn't pronounce these sounds right but was not in the slightest bit worried as she has an excellent vocabluary and is an enthusiastic speaker. Parents in law claimed they can't understand what she says but I can understand her 95% of the time and no-one else has ever mentioned that they can't understand her, including the playgroup that she attends 4 mornings a week. Should I follow my gut instinct to ignore them, or should I take it more seriously? Don't want to hinder her development. She sucks her thumb and wondered if that might make a difference?