I think treating Siri like a digital version of a Personal Assistant is going a bit far. It is a simple Voice Interface to a bunch of apps, local databases and cloud databases.
Nobody has ever said "Should I buy an Apple Watch for $750 or hire a PA for $35,000/year?".
I'm not a parent, so just speculating wildly, but there's a good chance your 3 year old has already cost more that $1750. Also, having met a few in my extended family I would doubt that a 3 year old not from London would actually know the time there, even if they could properly disambiguate the question. The phone seems like good value by that comparison
Nobody has ever said "Should I buy an Apple Watch for $750 or hire a PA for $35,000/year?".