Do nothing but let them grow. Learn their habits, learn how to take care of them so you don't kill them. I don't touch my plants until I've had them at least a year. If you must dig them up, be as careful as possible and do it before buds break. (That said, I have collected maples by pulling them out of the ground like weeds, so... Fragile, they are not.)

Then, when you've learned your maples and they're thriving in captivity, put a tourniquet around the tap root to encourage the plant to send out more surface roots. Don't cut it off, let the plant take care of itself. Let them grow in decent-sized pots, put on thickness in the trunk. Neither of those babies is anywhere big enough to bonsai yet, and I don't think anyone ever regretted letting a plant grow another year before chopping it.

Maples do not have tap roots :-)) In fact, they are notoriously shallowly rooted. If your salvaged tree is growing from a cut down stump, what looks like a tap root is really just the connection between the shoot and the rest of the cut down tree's root system (which was very much still alive, otherwise would not have produced your salvaged shoot).

Right now - January - is an excellent time to pot up maple shoots or seedlings, especially on the mild winter west coast.

I don't do bonsai per se but I do grow a big collection of JM's in containers which is really very much like bonsai without a lot of pruning and training :-) January is the time I do 99% of my repotting and root pruning, while the trees are still very firmly dormant.

All plants that grow from seed have a tap root when they're young. Most we don't think of as having taps have just outgrown them as they age and establish themselves. My maples have lost theirs by a year old, tops, but I have purchased young plants that still retained them. Usually, it's a sign they're competing with grass for water, or they're just plain not being cared for.

That these plants haven't is indicative of some less than stellar growing conditions.

There is a big difference between an anchorage root and a tap root!! All plants do NOT develop a tap root when young but do develop a primary anchorage root. THis is just basic botany.

And if the OP's maple shoot was growing out of a cut down tree stump, it very likely has neither a tap root nor a real root system at all but is still accessing the tree's root system for sustenance.

Surely you know better the behavior of dicot seedlings than the field of botany.

This is, as you said, basic botany.

Start a maple from seed, if you don't believe me (or the botanists). Not a grafted rootstock, a seed. Muck about with the roots, observe what they do. They're not as significant as oaks, but maples are dicots and they do have tap roots as seedlings and juveniles. These tap roots give way to the normal surface roots we know and (in bonsai) cultivate.

The shoot doesn't have a tap root, just a root that'll be a big bother and should be dealt with using the same sorts of techniques for much the same reasons. Heck, it's even using the root like a tuber.

The volunteer seedling evidently does have a tap root, which makes sense because the thing is probably only a couple of years old at the most. It hasn't developed its adventitious roots yet. If it was in grass, that may be a factor in it.

As Steven said all dicots start their life with a single tap root - that is one of the distinguishing feature of a dicot vs monocots - basic botany. Many plants will quickly produce adventitious roots from the base and the tap root either goes away or becomes indistinguishable from the other roots. Maple is one of them. I have grown quite a few (japanese) maples from seed and it definitely starts off with a typical tap root.