Never had a tree die from dog pee before and you could make up a mixture of chilli/garlic and water to spray your veggies for pests.

You can always use a bit of plastic mesh or wire to surround the trees if they are very small.
There are probably enough articles and books written on pests and gardens and such, to fill your house.
Best start a little at a time I think, it's all learning and sometimes the best results are ones you discover for yourselves.
I'd suggest you simply start with the main veggies you intend to grow, and also identify what climatic area you are in.
Then try to reasearch as much as you can - search engines are great
Don't think of it as an overwhelming task, you can often solve many questions by contacting your state's Department of Primary Industries or equivalent and they may have fact sheets.
Start simple, make notes, cross check, ask!! Most of all, just try things.
When you research on the internet, try to be specific to Australia and your State on
- the pests and diseases which may attack any specific fruit or veggie
- when the pests or diseases occur & if there are conditions they like or don't like
- if there are any companion plants that deter
- if there are any friendly bugs that are predators and can help
- organic remedies - if that's the way you want to go

- chemical - I mean pesticides and such - if that's the way you have to go.

Nothing's going to be entirely safe from pests, especially if you grow yummy veggies. Some things are as simple as watching and squashing a bug. Some things, maybe will never be a success for YOU or your area, but others will.

Just enjoy and have fun and I'm sure you'll find your son's delight enough encouragement.

The best organic way to prevent pests attacking your veggies is to grow them strong and healthy. Read up on companion planting, and grow flowers and herbs next to your veggies to either deter pests or attract insects that eat them.

If your veggies are attacked by caterpillars, aphids, snails, grasshoppers etc. the best remedy can be your thumb and forefinger followed by a boot :D

There are loads of books about organic gardening and organic ways to get rid of pests. Your local library should have a collection of them. Have a look at Jackie French's books. She lists a number of organic sprays that can be made from ingredients in your kitchen pantry.

Welcome to gardening and many years of enjoyment ahead :)

I was using eco-oil on the caterpillars but getting to both sides of the brocc and brusselsprouts was too much effort. Found it easier to allow some loss to the caterpillars, do a visual hunt every couple days and feed the ones i find to the chooks. My two girls, the chooks, are only too happy to eat them - and it is amusing, in a sick fashion, to watch the girls fight for them