It depends what you mean by 'child friendly'.

If you're thinking you can plant a garden which contains no risks for children, think again.

It's not just that a great many plants are toxic. There are all the spiky or thorny ones not to mention things like fruit trees and bushes which could give a child a bad tummy if they are allowed to run riot.

The answer is not to make the garden child friendly but to make the child garden friendly. Teach the child(ren) to understand that there are risks in the world and that learning is a valuable way to equip themselves to deal with those risks.

Make the three of four year old understand that not everything in the garden is good for them and they should leave alone anything they don't understand and you have a better chance of them saying 'No' to that pill they'll get offered at their first teenage disco.