We've only a short bit left of the summer now, and with you only seeing the kids EOW, how much wear would they get out of them this year if you don't let them take the clothes with them? 16 days? Instead of potentially 104 days of wear if they go back to the RP house, what a waste.
Think of the planet as well as the kids.
They may well have grown out of them by 2020.
Your £280 maintenance per month equates to 50p per day per child (not counting your DPs DSS) for the days that the RP has them.
If the RP buys each DC a £30 pair of trainers, £20 on trousers, £10 on 2 t shirts & £10 on a jumper, that's £140 on a couple of outfits for the 26-27 days of the month that she has to clothe them. Then she only has 25p per day per child from your DP to put a roof over their heads, feed them, buy school uniform, books, petrol for the school run etc, plus clothes the the other days.
I really think you're underestimating how much money it takes to raise DC.
Could you clothe, feed & house your one DC for 26-27 days a month if your DP only gave you £140 contribution?
Let his DC have THEIR clothes, put them first instead of being petty.