You would hope that the parents will separate amicably, and that each parent will be able to have the care of the children for a generous amount of time. Normally a parent would organise time with the extended family within his or her own time with the children.
Sometimes that is not possible. If the parent with care is effectively excluding the other parent, and it is not possible for the children to enjoy a relationship with their grandparents that way, then perhaps you might organise time with the children directly with the parent with care. If you are close enough geographically, you might be able to offer valuable assistance with childcare, eg before or after school, or in school holidays if the parent does not have sufficient time off. Or even just at weekends to give that parent a break.
Or it maybe that the parent with care is unwilling to agree time either for the other parent, or for you, the grandparents. If the worst comes to the worst you could consider an application to the court for a contact order. However, grandparents do not have an automatic right to make an application for a contact order - you first need to apply to the court for permission to apply for a contact order!
If you make such an application, the court will consider the following:
(a) the nature of the proposed application for an order;
(b) the applicant's connection with the child;
(c) Any risk of disruption to the child's life such that harm would be caused to the child;
(d) [applies to children in local authority care]
The court will always prioritise a parent's relationship over a grandparent's relationship. So, if it were your son who was the father, the court would try to sort his contact out first, if there was an application from him before the court.
Google Children Act 1989 and check out sections 8 and 10.
See if you can get a free consultation with a local family law solicitor.
I would advise you as grandparents who fear losing contact should be to approach the childs mother or father and explain that, no matter what the problems are between the parents, you as a grandparent do not intend to take sides but that you only wish to maintain contact with your grandchildren.