garnerlives:
her mother is still alive
What a bizarre thing to lie about. It’s a classic attention-seeking ploy.
It seems strange, too, that your children are not regularly seeing their grandmother, so they would KNOW she was still alive, because they were in regular contact.
At 4 and 6, a grandparent’s love is very important to a child and they can have a lot of fun being spoilt by them.
And the additional childcare assistance is usually gratefully accepted by an harassed parent…
Unless there’s a good reason for the children not seeing their grandparent - illness, Alzheimers, distance, etc.
Even stranger when it’s your ex’s own mother.
Nowt as queer as folk, Garner! 
Sounds as though you and your ex missus need to have a civilised chat on neutral territory, to try to sort out what’s best for the children, with a counsellor as mediator, if you think the discussion might degenerate into warfare! I think Relate have specialist counsellors for this.
If you know that won’t work at the moment and you have a good enough relationship with your ex mother-in-law and you think your children would benefit from seeing her, you can always take them to see her, if that would be possible.
Break ups are always so hard - on everyone.
Your children will be pretty switched on at their age and will probably be asking their mum why she lied to them! Children don’t miss - or forget - much!
I hope you manage to work something positive out.
