What helps - as has ever been the case - is being lucky in your timing.
One of my dc went into a shop as he was walking home with his mate and the chap behind the counter asked them "Either of you looking for some work?". Unusual, but the same applies to all the basic / unskilled jobs in cafes and pubs and bars that don't have HR departments and the like, you can be lucky if they just have a vacancy arise at the time you walk in to ask.
It also helps if you know lots of people and chat to people. If your 16 yr old only speaks to mates their own age they are unlikely to know anyone hiring. If you've encouraged them to take part in things and they are known to adults (eg sports coaches or their dance teachers or music teachers or Scout Leaders or Youth Club leaders) the circle of people who would put a word in for them, and also who might know of jobs is much wider
Experience of working with people - often through helping out with a younger group maybe at dance or drama or music or football or rugby, or being a Young Leader a Guides or Scouts, or volunteering at Park run, or helping out at the Community Cafe or any other volunteering in their teens.
Gaining qualifications - training as a referee, or as a lifeguard, or music grades or other niche skills you are willing to pass on.
Being willing to do the unglamorous work
Being willing to be flexible - hours, early starts or late finishes.