I have some kale growing in my raised bed which are getting infested with aphids. How do I get rid of those and secondly, are the kale leaves still edible (after cleaning them in hot water)?
First and foremost, do confirm they are aphids. Pictures would help us confirm.
Once you have been attacked by aphids, you are probably going to see them again. The real long term solution is to employ lady bugs. These guys will take care of the aphid problem quite well for you.
Stormy is right. Hot water is not a good idea, unless you are talking about using hot water after harvesting them. In that case, your Kale may not taste as good and you may lose some nutritional value. For cleaning, a thorough rinse with warm water and a scrub should be sufficient.
If you want to help the plants while they are still in the bed, I repeat, hot water is not good. You then need to try a very diluted soap solution in water, and spray vigorously. You might have to do this over multiple cycles.
Herbicides are not good, in general. The mechanical wash from your spray and the establishment of lady bugs over the long term for future harvests is best. You can actually buy lady bugs at stores, and must make some previsions for them to stay and be happy in your yard. In most regions, it is too late in the year (unless you are down under) for lady bugs as a long term solution, but it ought to be one of the first things you try in Spring.
Talk to local master gardeners, nurseries and/or agricultural colleges, should you have access for specific solutions.