Grandkids unusual/troublesome behaviour

Sounds like a separation issue more than anything sinister. Children of that age can't always, or don't know how to express their feelings where there's been a divorce or loss of sorts within a family. How they feel tends to be played out in what they do as it is upsetting for them. All kinds of behaviours can be displayed.

There's frustration, confusion and just a general lack of understanding as to what has occurred between mum and dad.
If you can calmly sit down with the 5 year old and try and ask him if he's unhappy about something as they're usually quite forthcoming. Interact with the children and at the same time see if you can spot anything that doesn't sit right as regards behaviour then ask why that's happening.

The " loss " in the home of either parent is traumatic to the child and all you can do is talk your way through why they're feeling the way they do.

I'm not sure what you can do tbh. You say dad had primary custody but do mam and dad have shared parental rights. If they do then mam can have them checked out by GP etc. and talk to school/nursery.

If you are not "friends" with dad, then there are clearly some other issues going on here. If there is tension in the home of any kind then children will react in different ways - for attention, out of distress, confusion etc etc.

If you cannot have a sensible discussion with his father then there really is little else you can do. Is the mother involved in his life still? Is she concerned? Can you discuss with her?

Presumably you have access to these children to observe their behaviour, so I am assuming you have contact with one parent at least. If the older child's behaviour is 'scary', school will pick up on it and discuss it with the parent (s). The poor toilet training is sadly increasing at school, but that too will be noted and followed up.
There is little you can do to get help for this child. other than discuss it with their parents and trust the school.

I would talk to your dd.

If anybody can do anything, it is the mother of the kids

Hithere But the father seems to be the primary carer so for some reason their mother is out of the picture.

Sounds like the kids have had a rough time. Children often act out their emotions as they do not have the emotional literacy to explain what they are feeling and why.
Rather than get angry or judgy, I would want to find out what is going on. Where is Mum? Is Dad a kind man? Are the childrens needs being crowded out by Mum and Dads issues? Is there any chance the child is being abused in any way including emotionally?
To cast all the blame onto a five year old and label their behavior as manipulative seems very harsh. Surely with all your training you have learnt about attachment and trauma issues with young children?
I agree they need proper assessment and help. I also think the adults, including you, must put aside your personal issues with each other and put the children's needs first.

KarinJune, make friends with Dad.

Attachment Disorder I would have thought too.
Good post Clare, I'm surprised that op didn't mention AD herself.

Habitual liar what a way to describe a child of five who is obviously suffering because of parents break up and situation. You don’t get on with the father who I gather has custody why doesn’t the mother?
Well no matter what has gone on every effort could be made now to improve to the atmosphere in the GC company. If the mood is bad the GC can pick it up.
I would be supportive not judgmental why pass the responsibility on to child welfare surely this should initially come from family.

The poor kid must be missing not having a mother around and at 5 would have no understanding why. I think this family needs help and fast,too. A five year old is not much more than a baby, don’t expect so much from them. A confused child can be aggressive, it is a defensive action. Just show love and understanding and try not to get angry.