I don't believe there are any existing products for that specific strap. However, I do have a DIY solution that can be just as easy, depending on your car seat model. This solution would work for my son's car seat and his booster seat both, but I can't tell from the picture you posted if it would work for your model.
If you search for "strap slides" or "strap sliders" you should be able to find products such as these:
Rounded metal slides
Metal slides with crossbar
They make them in a variety of sizes, so take a measurement of the width of the tightening strap and get one that's just that width. I think they're usually about 1 inch wide, but I'm not sure.
I would opt for the crossbar models. They end of the car seat tightening strap is usually fairly think, so the adjustable crossbar allows you to more easily feed the end through the slider.
You can also find plastic models with grooved crossbars that increase the tension, but many of them don't appear to be wide enough to fit the end of the strap. However, if your car seat model allows you to complete unfeed the tightening strap, the other end may not be as think and make these other slides the best bet:
The reason I say that is because to make these sliders effective, they need to provide sufficient tension. If they're too loose (because they needed to be to feed the thick strap end through them), then they won't stay in place and prevent your child from tampering with them.
If you must use one of the metal type slides, you may be able to increase the tension after adding the strap by wrapping some other material around the crossbar. My suggestion for this would be a strip of cut fabric that your can "roll" around the bar after a few manual wraps. Then, I would use some fabric glue to seal it in place. Note: This will make later removal of the tension slider difficult.
Once on, you just slide these things up towards the button/toggle that releases tension, and it'll help keep the straps from sliding even if the toggle is pressed. However, if your child is also putting a lot of pressure on the strap (such as leaning forward in earnest), then they may very well overpower the friction of the sliders.
I would add that most of the products do not come in solo packs, so you may need to buy them in packs of 10+. Or, you could cannibalize an unused adjustable strap (such as one for a camera strap, or Guitar Hero/Rock Band guitar strap).
An alternative method is using something that can attach to the strap and prevents the harness release toggle from being pushed/pulled by fitting underneath it. On my son's seats, if I were to attach a small alligator clip to the strap as close to the release mechanism as possible, then I'd be physically unable to push down the release toggle until the clip were removed.