I love Breaking Bad questions about symbolism, but fair warning: it's like opening Pandora's Box.
As you've rightly pointed out;
everything appear to have some meaning
...But there's often a lot of debate over their interpretations. It could be observed that in the deployment of narrative devices such as foreshadowing, recurring motifs and visual metaphor, a variety of connotations are often purposefully woven together, saturating objects with symbolism.
Vince Gilligan is quickly getting a reputation as the Stanley Kubrick of TV, so you're probably right to assume most profilmic elements are purposeful, and potentially loaded with symbolism.
Once a show can demonstrate that level of detail, it becomes impossible to stop searching for more things to decode. Breaking Bad has proven its capable of this time and time again.
With that in mind, here are a few options to explore:
First, lets look at the colour they've chosen: Yellow.
Yellow has long been used as a precursor to danger, or warning of threat and peril. In football (soccer to our American friends), a yellow card is a warning to desist; In Aviation, yellow signals caution ahead;and appropriately for a car, a yellow traffic signal is intended to alert drivers to an imminent change of circumstance.
Yellow, and its cousin Orange, are used frequently in television to signal danger, as pointed out by others.
Vince Gilligan is said to be engaging with this emergent TV trope; look at this compilation of Breaking Bad deaths and notice how many times orange or yellow features in the scene: almost always.
It's been said that this is such a prominent visual device, Gilligan even parodies it himself by having Ted Beneke 'attacked' by the most quintessentially orange thing out there: The Orange.
The fact that the car is prominent in scenes the prelude violent or dangerous circumstances would perhaps indicate that the car serves as a visual indicator to danger, flagged by its colour and supported by Gilligan's previous use of oranges/yellow.
TV Tropes also identify a character trend using a model called 'Blue and Orange Morality', in which they discuss characters which 'have a moral framework that is so utterly alien and foreign to human experience that we can't peg them as good or evil', and cite it as a popular narrative device. It is alleged the use of these colours throughout the show could be a reference to this, with other colours representing different themes:
Another interesting possibility of my own, is alluded to in your very title:
It is a remote 'control' car.
It could be said that the car represents control, as it is something manipulated by another entity to its own command. The use of a remote control car is an exertion of dominance over a foreign object, and furthermore the car is useless without the direction of its keeper.
Also; the car the Kid is using is a replacement supplied by Hank, after Marie purposefully runs over the original.
Marie destroys this in a fit of pique after he reminds her that she should be attending her 'meeting', thus visually destroying Hank's 'control' over her and the remote 'control' car.
The fact that the car is being 'played with' during the scene in which Hank and Walt are wrestling with their control over each other, is interesting. It's one 'power play' in the foreground mirroring another in the background.
Also; look what the kid is wearing...
Attention has already been drawn to the fact that the last child we saw wearing yellow was this unfortunate youth:
Finally; what is a 'remote controlled' car? Its a mimicry, a feeble replica of something much more powerful than itself that seeks to imitate its 'bigger version', its authentic original.
Is this not what Walter White is? is he not 'playing' at being a drug dealer, mirroring what others do as best possible? It's already highly discussed that Walt seems to adopt the characteristics of others around him, as though he doesn't really know what he's doing and is just copying those around him.
Could Walt not be the car himself? A smaller 'copy' of the real thing, trying his best to fool those around him? perhaps the car is a manifestation of Heisenberg, an avatar; leaving Walt to be the kid controlling it from a distance.
Any one of these theories, or all of them together, could be true: maybe none at all. That's the beauty of the show, and why
I love Breaking Bad questions about symbolism.