Measures like this should be reserved for cases where other efforts are impossible and/or have have been exhausted.
Good examples are animals, mentally impaired individuals, or criminals - all cases in which compliance through other means is likely to fail, for whatever reason.
If you have a child that's at a point where that would be the case, a LOT has gone wrong - or they have a serious physiological condition, in which case it may be a consideration. For everyone else, good education, parental care, and heck even something like a clothes tag are infinitely preferable to surgically implanted devices. If you feel you need to control and check your child's movement to such a degree, perhaps the one who has something wrong with them isn't the child.
Security concerns are a serious matter, of course, and while still largely within your control (the majority of, say, kidnapping cases involve some sort of risky behavior) there are definitely cases where you could not have done anything better. Those are very, very rare, and at some point you simply have to accept that you cannot safeguard against everything. Again, of course, it's a different discussion if there are specific known risk factors (like if you're e.g. rich or powerful and there have been threats against your children, or whatever).