Landscaping - Shaky Landscaping Bricks

I just put some landscaping bricks around a particularly large section. They're flat bricks and I stacked them only 2 layers high, so I didn't bother to use any mortar or cement, but instead just stacked them and filled the area with mulch. It actually looks great right now. But my concern is that some of the bricks are a little bit wobbly - some are tight because I put them in tight, but they're very slightly different sizes so some of them have a little wobble to them. If you knock into them hard, or if the kids walk along the top of them (which they inevitably will), I feel like they'll quickly come loose. Once the mulch and dirt settles will they eventually tighten, or should I have used something to keep them in place? I'm also concerned that after a freeze & thaw they'll shift and not be straight anymore because of this.

As usual any advice would be greatly appreciated!