A year 1 kid does not get so sweaty their clothes need washed. Unless you are in some kind of forest school, how the hell is he getting muddy every day? Perhaps if he is spilling food every time he eats, that’s something you need to be working on, because I can assure you, most kids will not spill at every meal. A shirt is not generally “grubby” after a day of normal use. Wearing fresh trousers is not the same as cleaning your teeth, don’t pretend that it is.
A less active Year One child may not get sweaty, (especially if they’re driven to and from school with no risk of mud splashed trousers from scootering through puddles, no wet fields to cross, no adventure playground to run and climb in on the way home).
My son spends breaktimes playing chasing games like tag, running, getting muddy knees and grass stains. So even without the walks to and from school he gets a lot of exercise.
Then add all the grime from the adventure playground and the sweat from climbing cargo nets, going down slides, rolling on astroturf and bark etc (on our way home).
His tie and jumper normally have splashes of what I assume to be soup or paint, so yes they get washed too.
If clothes are dirty (sweat, mud splashes, grubby knees, food or paint) then yes I think it’s appropriate to wash them. I wouldn’t send him to school in dirty clothes any more than I’d send him without brushing his teeth or washing him!
Appreciate some year one children manage to keep their uniform clean enough to wear for a week. But many don’t! And the school itself asks they’re sent in a clean set each day.