I stayed at the CP Marlow and the HI Maidenhead with parents and a small child recently. The CP seemed to tick more boxes and I probably would have preferred it had it just been husband and myself alone, but the HI was just so much more enjoyable to stay in.
If you're going with kids to the CP Marlow try and get some guarantees that they'll get to use the facilities they advertise - we never got a sniff at the childrens' room or wii area as it doubles as a business centre and was out of bounds throughout the entire stay. The HI kids' room was pretty basic but at least our child got to make full use of it and had a blast with the other children. Similarly the pool felt sterile and had an unpleasant chlorine tang to the air, more like encroaching on a private fitness centre, at the CP. At the HI the pool was more like that at a leisure resort and the pervading smell was more inviting. You could actually use the saunas there too (unfit for purpose at the CP).
TV was way better at the HI also (Marlow still has old cathode ray monsters).
However, the Thames river walk to Marlow from the CP Marlow was stunning and this feature is the only reason I'd personally return there. And keep your receipts - we got charged twice for our night.