I'm with jimjams on this, and Starlight, and bochead and lots of others.
my 7 year old gets rewarded for wiping her bottom (or, more accurately, attempting to wipe her bottom)
my 5 year old gets rewarded for trying some of her vegetables without complaining (the without complaining is key).
my 5 year old is perfectly happy whn another child achieves something thye have ben working towards - she might have a twinge of envy, if the 'prize' is somethign coveted, but she is absolutely over the moon that the child has worked hard (for them) and achieved what was set.
my 7 year old (with complex learning difficulties) understands differential rewards, and that what she is workign towards is not the same as what her sister is working towards. quite frankly, any NT child over the age of about 3 is capable of understanding this, so why the fuss?
we are not all the same. we do not find the same things easy/difficult. some people need extra help with bottom wiping, others need extra help to not scribble on books (be that an impulse issue, an ODD issue, or a sensory one), others still need help with not hitting (again, possibly an impulse issue, possibly a need for attention, possibly an interest in the varied responses that can ensue). my 5 year old does not need help with any of the above, but does need help with confidence during school swimming, so she is on a (school) reward system for that - are the other children bothered that she gets a sticker for getting into the pool without crying? do they think they should have one too? nope. they are pleased for her when she is able to achieve her (considerably smaller than theirs, in that lesson) targets, just as she is thrilled for the girl who gets the sticker for sitting still, or the boy who gets the sticker for good phonic work that day - stuff that comes easily to her.