Edit: I stand corrected, titanium dioxide was, maybe still is, but not considered safe.
Si02 is just silica/quartz, and is the most common type of sand. It's usually an anti-caking agent but... it's sand. You are almost certainly getting some SiO2 any time you eat a vegetable. It is also generally recognized as safe.
It's the size of the particle that matters more than what it's made of
There may be longer term effects, but I'd imagine we would have identified more of them by now, considering how severely dangerous silicosis is.
And double blind tests aren't hard to do.
If you're going to feed kids candy that is colored with agent A you need to make identical candy colored with agent B so the kids can't tell them apart. You can test A vs B but not A vs no color. (Though maybe you can have the kids eat the candy while blindfolded)
If you're going to test the influence of sugar you could use sugar for candy A and some combination of Sucralose, Ace-K, aspartame and some strange carbohydrate like mannitol or xylitol in candy B and get pretty close in terms of taste and mouthfeel but in A/B tests people will notice the difference.
Then people will wonder if those other substances cause problems and none of these address the possibility that the sensory pleasure of junk food makes kids squee and lose self control.
I knew it from 'Strawberry Milk' which made me sneeze and feel strange long ago. Since then I'm avoiding E122&E123. But haven't seen it that much in the last years. Still wouldn't want to have it. It's cheap stuff to upsell cheap products. Mix/blend some real strawberries into your milk/yoghurt, or use organic syrup, marmalade.