There is an excellent redback antivenene nowadays, and not all bites envenom the victim much. I have had several such bites, and suffered nothing worse than nausea and a headache. On the other hand, a whitetail spider bite caused me a lot of pain, ulcerated badly and annoyed me for months. Probably varies with the spider, the level of envenomation and the state of health of the victim. A little kid with fine delicate skin will likely take more damage than a big tough adult.
I certainly don’t take redback bites lightly, having seen some very nasty ones. I remember a newspaper article from a few years back concerning a big redback found on the back of a filing cabinet in an office in Perth, which had trapped in its web a young dugite snake (a thoroughly poisonous animal in its own right) which the spider had bitten and killed. I would not care to have worked in that office!
For the Americans among us: our Redback spider is Latrodectus hasseltii, similar to your Black Widow spider and New Zealand’s katipo. There are several South American species, and I think even a European one. The females have the more deadly venom, and often eat the males after mating, hence their common name of Widow Spiders.