M.P.
When she asks for attention are you often too busy at the moment? I see this as a common problem with my grandchildren. When they need/want something is often at an inopportune time. I could see a child then saying what they've been told.
I suggest it's important to validate her sad feelings. Let her cry. Comfort her. Eventually she'll get over feeling sad as long as she knows it's OK to feel sad. I don't know how to explain it but it works. When we tell children they shouldn't feel sad or shouldn't cry it makes them feel even more sad and like crying. They need to know that their feelings are a OK. When we tell them in action or words that the crying is unacceptable they then feel that they are unacceptable. Feelings are a part of who we are.
My suggestion then is to give her a hug when she cries. Ask her what's wrong and listen to her answer without judgment. Same for your son. Sympathize. "I hear you don't want to brush your teeth. It's tough but I want you to have healthy teeth and so you must brush." or "I see brushing your teeth makes you angry. You don't like me to tell you what to do. It's OK to be angry but you still have to brush your teeth." That sort of thing.
A phrase I learned while in counseling with my daughter was "it's tough to be you" said in a sympathetic tone of voice. I use if fairly often when I can't think of anything else to say and it seems to help.
I also suggest you read the book, How to Talk so You Kid will listen and Listen so your Kid will TAlk by Mazliesh and Faber.