SFAIK the Code calls for (a) access to the building to be refused (i.e. you can’t come in) and (b) “reasonable efforts to be made” to delay the departure of any child answering the description of the missing child.
Presumably the object will be to delay departure until it can be verified that the child is not the missing child, and presumably in most cases this can be done fairly quickly, since the person who reported the child missing is still on the premises. Those who insist on leaving can be directed to just one exit door, where there is located someone who will recognise the missing child.
Another point to note is that the legal enforceability of a protocol like this is only a part - and, typically, a relatively small part - of its operation and efficacy. Most people will be motivated to respect the code and co-operate with it even if not legally obliged to do so, and those who distinguish themselves by insisting on leaving without co-operating can expect to be noted and remembered (and, these days, probably photographed, and perhaps followed and their car registration noted) which may deter them.
But the bottom line is, yeah, in the absence of leglsiation to authorise it, a shopkeeper can’t forcibly detain you against your will in this circumstance.