Name changed for this and posting here for traffic.
My two kids (age 8 and 10) are frequently upset by the situation they find themselves in at their dad's gf's. The most recent thing is that they are upset that the gf always buys them clothes that don't fit and in their words "aren't as nice as the other kids'" (meaning the gf's kids.) she buys all their clothes, not their dad. No idea why.
This came about when discussing a party invitation and them being upset that if they are with him for contact on the day of a party then they have to go in scruffy, badly fitting clothes as this is what always happens. I feel so sad for them as they are so unhappy.
The clothes provided for them there are not what I would buy for them (not a problem as variety is good) but are mainly badly fitting, stained or have holes in them, the underwear stained. I want to cry when they come back home as they look so unkempt and uncared for. Their hair is always in ragged knots and unwashed. I despair. They are old enough to brush their own hair but claim they can never find a brush. The gf puts their clothes out for them to wear and they don't get a choice in outfit. They wear some very odd combos!
They have both told me the gf says she treats all the 8 kids (yes, I know, 8) the same yet they say she doesn't at all. The other kids get much nicer clothes.
I don't think she should have to be buying them clothes and shoes at all as is up to their dad but this is the way they seem to do things.
I have previously tried sending a change of clothes with them to come home in but have been told that they don't have time to get changed before coming home. Sending clothes with them for contact on weekends or holidays results in them going missing, being ruined or not being returned so in the end I refused to send stuff. Shoes shoes frequently don't get returned and they have to go to school in trainers. It's s nightmare.
Something else that has come to light is that the gf insists on them travelling in her car to places rather than in their dad's. They obviously have to take 2 cars with all those kids. Why would she do this? It makes me think she's stopping them from having time alone with them which is something she has admitted she doesn't like them doing. The kids are desperate for one on one time with their dad and never get it. She even insists on accompanying them home (in 2 cars) after mid week contact. Apparently she likes to say goodbye to them 
Is it so wrong of me to find this situation unacceptable and want to do something to change it? I can't talk to my stbexh as he is abusive (to the point I did have a restraining order against him at one point) and he'd tell me to fuck off and mind my own business. I hate seeing my kids so unhappy but don't know what I can do. It's got to the point my eldest wants him to leave the gf because there are so many issues. All those kids in a 3 bed house for a start 
Social services have been involved in the past and weren't interested. The kids won't talk to their dad as he tells the gf everything and they are really worried about upsetting either dad or the gf. I feel so helpless to make things better for them. (Contact is court ordered btw.)
Any advice or experience welcome.