The risk is simply not worth of it, and especially not if you dive with children.
- Not only you can suffer a barotrauma as Jon wrote when surfacing (or changing the depth during the dive) without exhaling adequate volume of air, but ...
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- ... you also risk to choke when trying to take your breath, which unlike at the surface can be fatal at depth.
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- Another reason to avoid it is that by artificially prolonging the dive, and by inhaling air of higher than atmospheric pressure, you increase the risk of DCS (bents / decompression accident due to Nitrogen bubbles released by the tissue when depressurizing during surfacing).
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- The next serious risk factor is that by just re-filling your lungs without proper breath-up, you change the CO₂/O₂ ratio in your lungs and make so very difficult to the brain to recognize when you really need to breath. It means higher risk of hypoxic blackout.
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- Having higher volume of pressured air in lungs, with high level of CO₂ seriously increases the risk of CO₂ blackout - CO₂ is toxic at higher levels, and since you manage to get more air (hence also more CO₂) into your lungs without ventilating, and also staying longer submerged, you expose you much more to the toxic effect of CO₂ than normally.
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- By keeping the bucket open for a longer period can be dangerous - especially in lakes or caverns, or in seismically active locations, there may be methane or other gases escaping from the bottom, and can get into your bucket. Inhaling such toxic gas from pockets in caverns or from Kesson bells without purging it previously with clean air, is one of very common reasons of fatal accidents.
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- By the maneuver and the stress it adds, you may actually lose more than you gain, and will probably not manage to prolong the dive seriously anyway.
If the pure freediving is not satisfactory enough to you, I suggest that instead of taking this risk, you sign together with your son to a Scuba course; learn more about the theory; and drill the proper and safe way of diving.
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