There is no real way of keeping wasps out of your garden and given the job they do of pollenating plants you really need them there. If you had a small baby I'd suggest netting the pram but at 20 months old your toddler will be into everything. Unless he annoys a wasp it is unlikely to sting him.

I used a hanging glass jar thing last year. You fill it with sugary water and the wasps go into it from underneath. Then they can't get out and eventually drown. We caught hundreds of the blighters last year! The glass jar thing was from a large garden centre and they are quite easily available.

Might be a bit of a nuisance at times, but wasps are actually very beneficial insects and are invaluable to the environment and especially to the gardener!! in helping to reduce other insect pest populations. So unless they really are a direct threat to you or your family's well being I see no reason on earth to destroy them.