Obviously nobody can guess what another person is feeling with 100% accuracy all of the time. "I am wondering..." is a useful little phrase for gently speculating about what you child might be feeling and why. So if your child is looking glum and you are not entirely sure why, it is safe and non-rattling for your child to hear you 'wonder' what might be going on for them as, after all, if you are wrong, you can be corrected. It's so much better than, you must be feeling X because of Y - as mostly people would feel somewhat irritated or worse still, disempowered in the face of another declaring this -especially if the statement is incorrect. It's the feeling equivalent of putting words into another person's mouth.
This, "I'm wondering...." phrase works well as it serves a few purposes when you use it:
It gives you an opportunity to let your child know you have noticed something is going on for them.
It gives you and opportunity to speculate about what they might be feeling and why they might be feeling that way. This in turn can give your child the opportunity to correct you if you are wrong and hopefully initiate a two-way conversation.
It helps your child's emotional literacy as you will be exposing them to different words for emotions and what could be a reasonable cause of that emotion. e.g. 'I am wondering if you are feeling impatient because I have had to make you wait a while before we go to the park.'
It's good to use in times when your child is experiencing very uncomfortable feelings and expressing them ineffectively as you are demonstrating the vocalisation of how your child might be feeling.
It can also be used as a successful tool in behaviour management, linking negative behaviours to the underlying emotions that might have triggered them. e.g. I am wondering if you are refusing to put on your shoes because you are still a bit angry with me for asking you to stop painting because we were in a hurry.'
I personally use it with the children I teach as they so often cannot link their reactions and behaviours to the underlying uncomfortable feeling or what caused it. Instead they are just reverting to defensive, often aggressive. behaviours as an inappropriate expression of their distressing emotions. " I am wondering..." helps them start to make the links between their underlying emotions, what happened, their thoughts and their behaviour.