Will baby milk cartons or yoghurt pots explode in a freight hold of a plane?

If you mean, will the air pressure in the hold drop enough to make the
containers' internal pressures enough to explode, the answer is no.
I've flown many times with things in my checked baggage such as
shaving cream in pressurised cans. It never explodes. Even my
toothpaste always arrives intact. The cargo compartments of passenger
jets are kept sufficiently pressurised to allow impromptu entry by the
crew, as well as to keep pets in mobile kennels alive. There's plenty
of pressure to keep milk cartons, etc., from bursting.

I used to fly a lot on commercial airliners, and nothing in my checked
baggage ever exploded. Twice I had some minor leakage, though: once
from a can of spray deodorant, and once from a bottle of hair
conditioner. It's best to put such things inside plastic bags. But
milk and yogurt? I would worry about spoilage more than explosion.

"The cargo compartments of passenger jets are kept sufficiently pressurised to
allow impromptu entry by the crew, as well as to keep pets in mobile
kennels alive."

Passenger and commercial airplanes' cargo holds may be pressurized,
but there may be freight airplanes with unpressurized holds. I don't
know that that would necessarily cause milk cartons to explode, but I
have heard of cases of things like fountain pen ink cartridges or
tubes of artists' paint leaking or rupturing when inadvertantly
shipped via air in an unpressurized hold...

Thanks. I didn't know that.