Every seller of a child's pony believes, or says, that the pony is safe. The truth is that a viewing shows you the pony on its best behaviour, in its yard, handled by the people it knows, often after it has been worked or prepared for your arrival. A pony that is safe for a child proves it differently: it behaves the same way repeatedly, in different circumstances, with unfamiliar small people doing the handling. Your job as a buyer is to manufacture those circumstances before money changes hands.
Visit more than once, and vary the visit
Go back at least twice, at different times of day, and once with little warning. A pony that is docile at an afternoon viewing but sharp, bargy, or hard to catch at a morning visit is telling you the truth the second time. Ask to see the pony caught from the field rather than presented ready in a stable — plenty of "safe" ponies cannot be caught by a stranger, and a child certainly cannot learn on a pony an adult cannot catch. Watch the whole routine: feet picked up, girth tightened, bridle put on. Discomfort or resentment during these jobs shows up with children long before it shows under saddle.
Put your child in the picture
A pony handled beautifully by its adult owner proves nothing about children. Children are smaller, noisier, less predictable, and they flap, squeal, and drop things. With the seller's agreement and everyone wearing hats, let your child lead the pony in hand, groom its shoulder, and — if they ride — ride it themselves rather than only watching the seller's teenager demonstrate. Ride in an open space, not only the arena the pony knows by heart, and include the provocations of real life if you can: a dog nearby, a plastic bag moving in the hedge, a gate opening. You are not trying to frighten the pony; you are watching how it reacts to the unexpected. The genuinely safe pony startles and recovers, or barely reacts at all. The risky one is still wound up ten minutes later.
The sedation question
Some quiet ponies are quiet because of chemistry. Signs worth noticing are a pony that seems drowsy out of proportion to the weather and workload: a drooping lower lip, a half-closed dull eye, a head carried low, sweating without work, or a pony that is strangely indifferent to things that should interest it. None of these proves anything on its own, and tired ponies exist. The buyer's protection is a pre-purchase veterinary exam with a blood sample taken and stored, so it can be tested if the pony's personality changes dramatically in the first week at home. A seller who refuses a vetting, or refuses the blood sample, has answered your question about the pony in a different way.
History you can check
Ask for the boring evidence. How long has the pony done the job it is being sold for? Can you speak to the previous home, the pony club instructor, or the farrier who has shod it for years? Does its passport age match the teeth, and does the pony in front of you match its passport description? A pony with a history of children's work — passed from family to family as each child outgrows it — is the best predictor of the next child being safe. A pony with gaps, a fresh import story, or a sudden career change into children's work at an advanced age deserves much harder questions.
Making the decision
If the pony passes repeated visits, child handling, open-space riding, and an independent vetting, you have done everything a buyer can — line the professionals up before you need them, because the vetting has to happen where the pony lives, not where you live. KinsLe's Pet Services Directory lets you find and connect with a vet local to the pony for that pre-purchase exam, and the same directory becomes your route to farriers and instructors once the pony is home and the assessment — your child's first season on it — begins.