Can you place the pot(s) just prior to the opening? If you can then you won't be restricted to the 8" that is mentioned. A suitable plant for the aspect in a large pot should stop the toddlers getting past. Something heavy that they can't topple over.
One of those wooden corner planters with a two sided trellis attached? Heavy enough a child can't move it, but could easily be dragged aside by an adult?
This is one of the few occasions when I will recommend an artificial plant. There are some extremely convincing ones available these days and can come complete with pot.
A trowel in the hand is worth a thousand lost under a bush.
You don't need tall and thin plants, with a trellis you can add some climbers. Which depends on your light levels in each location, and the size of the soil in the bottom.
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A picture would be helpful.