grandchildren being manipulated by daughter

I posted a while ago about looking after our grandchildren, so the situation has moved on a little. Over the last 2 months we were looking after our grandchildren who came to us because their mother was addicted to diazepham and has some mental health issues anyway. She asked us to have the children who are 2 & 4 years old.
She doesn't want to talk to me despite us asking her if she wanted to come and visit the children, she preferred to see the children at my mothers house, she has seen them about 4 times over the last couple of months.

The social services who are pretty dire in our area I have to say dragged their feet and only started an assessment about 2 weeks ago - only gave the opinion that the children should stay here as agreed with the health visitor and other professionals, despite us requesting an order to say the children live here at this time, they didn't put one in place saying they hadn't yet assessed the situation.

During a contact with my mother, my daughter decided she wanted to take the children home, and did so. We were told by Social services to call the our of hours team and police should this happen, which we did, however the police did a welfare visit and decided that all was ok.

However she is now not allowing us to see or speak to the children at this point, she has done this before when we raised an issue of domestic abuse from her then partner, she only said that this was happening after they separated and then she said there was a lot of domestic abuse. We have been consistently a part of the children's life - we see them overnight at least once a week, and a few times in the week. My daughter really struggles with the children and has numerous times rang me in a crisis saying I had to come now to collect the children as she can't deal with them. When we don't just go she really neglects them.

She has recently made allegations about us abusing the children and historic abuse on her, we feel very hurt that she has done this but also concerned for her mental well being, we think that this was a ruse to try to get the children backfired.
The Social services seem to be taking the long road with this situation, but we miss the children and are very worried about them, it seems like a cruel thing to do - without warning to take them out of a secure environment where everyone agreed they should stay, and then not allow us to see them, we are worried about them.

We have always picked the pieces up for my daughter with the children and think that she will not cope for long with them.

The out of hours social work team seem a bit useless and I don't mind admitting we are both feeling very upset with our daughter who seems to make decisions on a whim which no one seems bothered to do anything about. On top of that we don't want to support her any more, we didn't have to have the children and we sure as hell didn't expect her to accuse us of abuse!! It just seems the social services don't take it seriously, suggesting that it is a relationship issue between us and her, it just surprises me that no one seems to be looking at what she is doing to her children.
Does anyone have similar experiences?