If you are thinking of sending food, check the official advice about bringing food, plant and animal products into Canada https://www.cbsa-asfc.gc.ca/services/fpa-apa/bringing-apporter-fpa-eng.html.
If you're worried about your child handling customs/immigration, one way to do it is to give your child a written/printed list of what they're bringing in to show to border control. In my experience, they will take your word for what you're bringing, but it can be a little intimidating to walk out of the secured arrivals area of the airport where they CAN stop you (but likely won't). Having a list to show and being able to say yes, I already declared what I'm bringing in may help your child walk through with confidence, although it's very unlikely they'll be stopped.
In general, it's possible to get UK products in Canada, although they may be expensive/difficult to source. If you don't know much about the host family, I'd go with sending things that are as local to you as possible. If they want to host a child, they're interested in that child's day to day life. If you want to send approved food products, perhaps put them in something like a drawstring bag (tartan or saltire or thistle/heather/etc. print, if you want a Scottish theme), rather than a hamper which will be awkward to carry.
I'd also consider something like a hardback/coffee-table picture book of your city or area - books are insanely expensive to order from the UK, but something nice to have to start a conversation with your child and as a thing for the host family to leave out for visitors/remember the visit and your child afterwards. Anything Scotland-themed will be pretty well received in Canada: even little things like badges or pencils or postcards.