The oo digraph
One of the things hardest things about reading in English is the inconsistency on how to pronounce letters, and especially letter combinations.
The digraph "oo" is a good example of this, as it has a long pronunciation (like boot, or room) and a short one (as in look, or stood).
How do you know which oo sound to use? You just have to know, but there is a rule which will help with many cases - if the word ends in k it is probably a short oo sound (like book), otherwise it is probably a long oo sound (like mood); BUT this rule doesn't always work (for example stood has a short oo sound).
Teaching oo phonics
I would suggest teaching your children that there are two oo sounds; one short and one long. When you are practising these, you can use flashcards to help differentiate which is which; for example Jolly Phonics uses a different flash card for each sound.
You can also just use one flashcard and ask them to tell you both the oo sounds.
Once they know both oo sounds, introduce words (via flashcards then reading) with them in.
When teaching oo words phonics they might need some help if they haven't seen the word before- you can say it is the long or short oo sound as a hint, or you can let them try and pronounce it themselves. For example if they pronounce "room" as with a short sound it will sound wrong and there is a good chance they will figure out the correct sound themselves - this happens to my son all the time.
Lists of oo sound words
Here's a list of words to help you in case you want to practice (make sure you can say the different oo sounds yourself before starting with your child - outside of a word it isn't always as easy as you would think!)
Short oo words
Short oo words
Long oo words