There are some reasons for me having them slightly longer, such as he can’t get them to school on time or to any after school activity, he doesn’t really wash or dress them well (he won’t for e.g brush DD’s hair) and his flat is quite shambolic, messy (not unhygienic just stuff everywhere).
The messy flat means absolutely nothing, you say it is hygienic. Many people live in mess. If he can't get them to school on time, you need to just inform school of what day his drop off is so he can get the moans. You are babying him here, if you want him to grow up then let him have he responsibility.
The washing/not brushing hair is subjective. When you mean he doesn't wash them well, what do you mean? Do you mean they have a simple wash and he brushes hair briefly? Or do you mean they come home with matted hair and filth all over them? Because the two are very different.
What you need to ask yourself is:
Is it just your standards are higher? Or is it actual concern for their safety/wellbeing? And is there a little bit of pearl clutching on your part?
Your title says he is turning them against you...I opened the thread thinking he had been dripping poison into their ear, not that he discusses contact in front of them (I agree not ideal!)
I would address your concerns with him and tell him it isn't on to do it in front of the kids, but ultimately if the kids want to spend more time with him and he keeps them safe then the reasons above don't amount to much. You do run the risk of your children blaming you.
Your children love you. They also love him. From the sounds of it you have both been muddling along nicely with contact until now.
Contact has to change as the years go by, that is the nature of it!