Yeah, but knowledge-wise, you (and most people commenting here) are probably above average, assuming we take “ordinary person” to mean someone with a generic level of education and critical-thinking skills — which is to say, not nearly enough.
Granted, most people may have learned about the broad strokes of World War I in school, but other than Franz Ferdinand (and maybe Gavrilo Princip), I doubt they know all that many of the key names or could describe his assassination if you quizzed them on it. When it was, where it happened, who Princip worked for, why Ferdinand was so important to keeping the peace, all basic factoids that — even for those who vaguely recollect them — don’t provide them a moment-by-moment “script” to work with telling them which sandwich shop Princip was at, when the car stopped, or how close he got when he opened fire, among other details. So, really, I don’t have high hopes of their presence changing much, other than random butterflies caused by them simply being back in 1914 — let alone what they do next. If anything, they’ll get themselves into further trouble, all without stopping Ferdinand’s assassination or World War I from still happening more or less on schedule.