I think you are going to have to be clearer on your 'three
nights on Vancouver Island' - do you mean in total including
driving time or are the 3 nights on the westcoast with 4 days
to spend exploring? Are you flying into U/T or driving from
the east coast?

There are quite a few toddler friendly places to stay in
Tofino and some gorgeous sandy beaches for a child to enjoy.

Since you are planning on being there in August you are going
to have to book your accomodation ASAP - that's the most
popular month for families.

Beaches on the west side of the Island are often quite rough-they are the last place I'd take a 2 yo.

Parksville/Qualicum on the east side-with it's vast shelves of sand and calm waters-would be much less worrying.

We took both our little girls there when they were toddlers
and we've taken our g'daughter when she was two - the water
is so cold they kept well above the high water mark so no
worrying was needed. But, yes, the east side of the island
is very kid friendly although it lacks the grandeur that
adult would enjoy.

either tofino or uculuelet are lovely. we stayed at a nice condo on chesterman beach. good view of the surfer's and beach. fully stocked kitchen. i don't know where you coming into but it is a pretty long drive (beautiful) from victoria to the west side of the island.
if you end up staying on the east side of the island...i would recommend that you check out ocean sands resort in parksville. get an ocean view condo. the ocean is very shallow there so swimming is nice and the water wasn't too cold. the tide goes out a long ways so you can't always swim but then is a good time to take kids looking at the tide pools.