Thanks for your question PolymathArtisan,
I read through your post a few times to make sure I understood the nature of the issue.
Ok - so firstly, a 'bosuns chair' is a completely different item of PPE compared to a 'harness'.
(PPE = personal protective equipment)
The term 'bosuns chair' (these days) means a rigid seat which is used to prevent the effects of suspension in a harness (which can be quite uncomfortable).
See attached photo.
Now I understand that you want to improvise a solution by simply configuring something from hand tied knots.
It costs nothing - and its cheap.
But, when someone climbs up a mast - there is a risk of a fall from height.
You don't mention the exact height that the 'child' or person has to climb - but I would imagine that a fall on to a deck of a boat would expose the person to sharp objects or various protrusions and rigging that could cause injury or death?
One of the best value simple design and easy to fit climbing harnesses on the market is made by 'Rock Empire' and the model name is 'Speedy'. it is fully adjustable (one size fits all) and the buckles are auto (self-locking). It is light weight and takes up almost no volume when stowed.
There will IGKT members on this site willing to provide info or explain how to configure an improvised harness - but, there is still the human factor and potential for error.
I know that some will be eager to offer their expertise - and no doubt they will.
But, consider that a 'Speedy' harness only costs around $50 US dollars - and you have peace of mind.
I will say that the metal buckles will need to be wiped down with anti corrosive/water displacer after being exposed to the salt water environment - but once wiped down it can be stowed in a bag to keep it dry and free of contamination.
This is food for thought for you... you may still wish to try to configure your own 'homebrew' improvised harness from rope - and if you do, it must be foolproof and not slip off easily from the user.
In the days before modern cheap harnesses were invented, sailor improvised with rope.
A
Portuguese Bowline was used by mountaineers and sailors at one time - the multiple loops can be splayed apart to create leg and waist loops (you need both leg and waist loops).
There a number of different improvised solutions you'll find on the internet - all require a degree of due diligence.
Mike Karash posted this video here:
https://www.youtube.com/watch?v=6EyfYyJkZss At 3:33 in the video, he describes how to improvise a harness from rope....might also be worth a look.