Sigh...here we go again...
What if officers get a call, then arrive at the property and no one is home? Will they just enter, then state afterwards that they received a tip about "dangerous, illegal weapons that posed a threat" in the home?
If there is nobody home, then there is no hurry - is there?
Post a cop or two to watch the place, while another goes to a judge to present the evidence. If the evidence is good enough to convince the judge that the search is "reasonable", then the issue is resolved and they can go search promptly. If it's not good enough for the judge, then it's "unreasonable" and they're not allowed to search.
What if a child answers the door, or someone too young to give consent? Will the officers simply walk away and "leave a minor alone in a house with firearms"?
If the evidence is enough to convince a judge - and an experienced cop should have a good sense of whether it is or not - they can act accordingly to search.
You're also complicating the question with the red herring of whether a child should be left home alone at all. That's a whole different discussion.
What if the person who answers the door consents, but isn't the property owner?
Then perhaps they can invite the person to step out, while a warrant is obtained. So long as there's time to get the issue documented with a judge's signature, do so. What's the rush?
What if the tenant of a rented property consents?
The tenant, for most legal purposes, has the authority to grant permission.
Does the landlord also need to consent,
Only if the search involves an area not otherwise under the authority of a tenant.
or will the police give amnesty to him as well for any of his property or any zoning laws or code violations they might "stumble across".
Well, ya see, that's an under-discussed part of this whole issue: while a verbal "we won't prosecute" is given, courts have broadly given consent to cops to LIE pursuant to their activities, and have also broadly given consent to cops to act on any other illegalities they reasonably stumble across pursuant to their reasonable search (hence the Constitution's insistence that a warrant be to search a
particular place for
particular things).
THINK about the issue. Don't just make up wild/scary-sounding stuff and go "oh, but what about THIS that I haven't really thought about?" It all revolves around what constitutes a REASONABLE search: why is a search being done? for what? where? when? why now? what's the rush? is there time to convince a judge it's reasonable BEFORE searching?