We bit the bullet and went for the fitted Volvo covers, extremely expensive I know!!
The good news is they are high quality and very well fitted with access to seat belt points etc and even allow the seats to be folded or part folded with them on.
On our crv we had a generic none fitted cover but it did cause difficulties. We carry one of our dogs in a cage on the back seat so want to protect the leather but when we carried a third passenger as wel,l we had to remove the cover to get access to the belt anchor points.
If a cheaper cover suits your needs, go for it. I wouldn't have paid the Volvo price just for the sake of it, only because it was what we needed.
Sorry, don't know the answer re booster seats, suggest asking your dealer.
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MY 15 XC60 D4 FWD SE Lux in red with power tailgate, f&r parking sensors, privacy glass, oh and beige interior which I just luv! -
GONE BUT NOT FORGOTTEN!!