If they have a ballast chamber, adding 20kg of ballast to the base of the speaker should make them stable enough for a small child to be unable to knock them over.

I assume your speakers have threaded inserts that you can use to spike them. If so there is another approach that would take a little effort but would be very stable.

You can create a plinth from a piece of fairly thick (say 1.5-2") hard wood.
Drill four holes to line up with the spike inserts.
Get some appropriately sized nuts and bolts to pass through the plinth and secure it to the speaker.
Use a router or large drill bit to remove enough material from the underside so each nut is flush inside the plinth.

If the plinth has double the area of the speaker base it will be far harder to topple as the speaker will need to be lifted to be toppled as opposed to just tipped. The larger the plinth the harder it will be to topple

Another variation if you are inclined would be to drill into the floor and use a bolt through one of the threaded inserts to secure the speaker to the floor.