well i havent washed my dd's hair yet and she will be one next week :) its not smelly or dirty, it has been rinsed a handful of times when she has been bathed but then she has had maybe 5 baths since she was born? she gets a good wipe down when i change her nappy and a wipe with a damp flannel if she needs it, but otherwise she is fine.
my ds4 who is 3 is bathed twice a week? sometimes more, sometimes less, same for my 6 and 9yr old, i wash the 6 and 9 yr olds hair once a week or sometimes once a fortnight, sometimes less, depending on if it needs it.
the 3 yr old has his hair washed infrequently as it doesnt need it. he has fine baby hair and it doesnt get greasy or smelly. it gets wet when he has a bath but not washed with shampoo very often.
the 6 and 9 yr old gets wet at bathtime but only washed with shampoo once a week or so.
my 12yr old has a shower everyday, this has started in the last 6mths or so as he got a bit older and is starting to go through puberty and so needs to shower every day and he has long very thick hair that is now starting to get greasy so it needs washing.
i wash my hair twice a week, its fine and doesnt need it anymore than that. i shower every day, or have a strip wash with just water no soap as i am extremely allergic.
we are all clean and not smelly. you dont have to use soap and shampoo to get clean, especially younger children.
re the wearing pants things, i dont see it as a big deal, i didnt wear pants when younger, just changed my trousers everyday as the op does wiht her dd. you dont need to wear pants, i only started as after having children i got more umm discharge and so it is more comfortable to wear pants but at 5yrs old i doubt the dd has much if any discharge and so there is no real need to wear pants.
it sounds like the school are on board with the sensory issues which is good, you could always try some of the links on here to the pants with no seams as they look good. and as long as her hair gets wet say once or twice a week and is brushed daily then i am sure its fine :)