What are the drawbacks to having the bathtub on a different floor from the bedrooms?

Children, especially young ones (diaper/nappy age), can often need a bath in the middle of the night to clean up from accidents. Turning on the lights, running a tub, and cleaning up the kid are enough hassle at 2am without having to also make a trip down and up the stairs.

Plus 1 for having the tub on the same floor as the bedrooms.

In fancy construction, I believe that the tub is usually placed in the "master bathroom" under the assumption that it would be mainly used by one of the occupants of that room. A smaller downstairs bathroom would mainly be used by guests, children, or elderly for whom a dedicated shower (no tub with high stepover) is the appropriate and much, much safer bathing facility.

EDIT In my neighborhood (1970 tract development of 270 single story) 40% of houses have 2.5 baths, 60% 2 baths. Originally all had a shower only in the master bath which was smaller than a central hall bath which had a tub-shower with 14 inch step-over. There has been a lot of remodeling and one of the things some people do is enlarge the master bath and add a free-standing tub. We have not remodeled extensively and neither my wife nor I have ever used a tub for bathing. I had to hot soak a lesion once in 42 years. I use the tub to wash our 20 lb dog using a hand held shower.

I now shower in the tub and have installed grab bars as an absolutely essential aid to getting in and out, but I know many people my age (upper 70s) who could not do so. There is actually a system for cutting a passageway through a tub as a cheaper alternative to removal and conversion to a shower.

I think the first floor location for a tub has advantages, but if all the bedrooms and the largest bathroom are on the second floor, then probably the tub should be there.

If you plan to stay in the house until you are old, consider what amenities you'd like to have when stairs become difficult, impossible, or risky for your aging self to use. Stair lifts have their limits, and having to move out of your house due to factors you built into it is annoying, if you would otherwise prefer to stay. In some cases this may lead you to choose to build a single-story house rather than a two-story house.

If you don't have long-term plans, make your choices as you will.

There are many "common things" that either never made sense, made sense only in the age of "hired help" or "roving bandits," or that only are "common" in one part of the world and not others. Try to look at them with a fresh eye when you get to build the house, rather than following "the norm" blindly.

To wit:

• Bedrooms on the second floor - harder to get out in the case of a fire, which you are already at disadvantage for from being asleep. Second floor views are generally better for the living spaces. Bands of marauding peasants breaking in downstairs and being held off at the stairwell by sword-wielding inhabitants is a bit outdated as reasons go, but there you are.
• "Laundry in the basement" = sure, when it goes down the chute you can't now have for fire regulations to the laundress you don't employ who gets to carry it back upstairs. The bulk of laundry is generated at and used in bedrooms, and the laundry processing should be near those and the bathroom(s) near them for efficient plumbing and not dragging most of the laundry around the house needlessly. In my area second floor bedrooms and basement laundry are the norm. It's nuts. Quiet machines, sound insulation, and/or scheduling use deal with the "noise of the machines while I'm sleeping" reason sometimes trotted out for locating them inconveniently. In other places, laundry in the bathroom is the norm.
• Who the heck decreed that shower/tub must share the room that the toilet/WC is in? They are both plumbing, and plumbing inside the house is a relatively new concept that humans are very slowly adapting to, so they got thrown in together when running those new-fangled pipe things into the existing house. Then that carried over into new houses. They are not the same, and making use of one block the other is at times VERY inconvenient. The privy/outhouse did not have a bathtub in it...
• No doubt other things that are not coming to mind at the moment...