The problem is that water removes heat from a body 15 times faster than
air. When you feel cold, it's because the RATE of heat loss from your
body is high. We're perfectly comfortable in 75 degree F air, but we'll
feel cold in 75 degree water until we get used to it because the rate of
heat loss to water is so much higher. Water sucks up heat like a
sponge. Once the blood vessels in our extremities (feet, hands) and
just under our skin contract to reduce heat loss, and our skin
temperature cools down to 75 deg. F, there's much less temperature drop
between our skin and the water, and so the rate of heat loss drops
precipitously. That's when we say "we've gotten USED TO the water
temperature", and it no longer feels cold to us.
In order to feel comfortable, your daughter is essentially making the
water temperature the same as her skin temperature. With no temperature
difference, there's no heat loss, and she doesn't feel cold. But, the
higher temperature of the water is resulting in heat loss to the room,
causing the room to warm up too much. It's like you have a 2000 pound
naked person laying on the floor in that room 24/7. The heat loss from
their body is sufficient to warm up the room.
I would buy an arctic sleeping bag, unzip it and lay it out flat on top
of the water bed bag. Then, make the bed up normally over that sleeping
bag. That will greatly reduce the heat loss from your daughter's body
to the water, and she won't feel cold even at lower water temperatures
in the bag. The insulation in the sleeping bag will reduce the rate of
heat loss from her body sufficiently so that she doesn't feel cold, even
with much lower water temperatures.
I've heard of two-person sleeping bags which you might consider buying
if it's a large bed.