Probably not a controlled study, so they'll miss confounders: poor parenting may be the root of both excessive screen time and bad nutrition etc. But generally it seems to be a good idea to keep screen time for small children low, and later monitor and adjust for effects.
You could at least monitor the nutrition of children before you insinuate that smartphones make them fat. When you want to identify causal influence, it comes down to collecting data on the counterfactuals.