Thank you for your question
My name is Clare
I shall do my best to help you but I need some further details first
How old are the children?
Are you aware that there is an extra charge for a phone call?
Were they in any way financially reliant on her - and did she say why she was leaving them out?
The children left out can certainly attempt to challenge the Will - but it is not likely that they will be successful unless they can show that she was supporting them financially; or that undue influence was brought to bear on her to exclude them.
I hope that this is of assistance = please ask if you need further details
If you think that she may have been influenced to change the Will then you can challenge the Will
You have to do so within 6 months of the grant of Probate