Even though Water Deer fawns are relatively small.I would not have thought that either corvids or kites would be much of a threat.
Unless they have a resident pair of Ravens, the only corvid which could possibly be of any threat would be Carrion Crows and I doubt it...
I have seen a Red Kite take a Wood Pigeon, but the likelihood of them taking fawns is remote. Kite numbers are artifically high in some areas as a result of public feeding. The are beautiful but dirty birds (hence the old English slang name of Sh*te Hawk, due to their scavenging on village middens), so they are certainly a risk to paddock housed carnivores if they drop their rotting food into open paddocks. Other than that their population density certainly reduces numbers of other birds of prey, owls and farmland birds like plovers, but they are not usually a direct threat to live animals.
If the Whipsnade site is no longer proof against predators like Foxes and Badgers, then these are very definitely a risk to young ungulates.