Erana said:
...Yes, one of the things I would like to do in life is illustrate children's books.
Needs more detail on the publication before you can decide.
If it's a kid's book in the Charlie and Lola style, it's fairly simple. Getting the content right in terms of recognisability for each character is more important than factual correctness. I wouldn't necessarily portray them as either, areas of flat(ish) colour with distinct profiles would work best. Maybe small areas on the outline that imply feathers for some, but only to provide a way to differentiate characters more easily.
Breaking from reality is acceptable if it makes the characters easier to identify with.
If it's a kid's science book, then both and it's tricky. Current science say they have feathers, but it's still not ingrained in popular culture. Without some kind of explanation, even if just a 'did you know' cutout box near the beginning, picturing the dinos as feathered wil garner a lot of scorn from people will little knowledge but a lot of voices.
More importantly, we only know that some dinsoaurs were feathered, not necessarily all of them or what the total layout of the feathers was. Were they downy and thick like most ground dwelling birds or did they have proper, aerofoil shaped quills? It's a bit tricky.
Even more, how will do you decide the markings? It's hard to know whether to go with a more believable mottled colour scheme or over saturated brights would work best. 'Proper' camoflague would certainly appear more credible to most adults, but might not hold the eyes of a young child very long. It's also hard to make a picture striking or lead a viewers eyes without any areas of strong contrast, more over it would ignore that some current predators are bright orange and stripey.
Brighter schemes would make sense from a composition point of view, but might lsoe credibility for the sake of being striking.
Anyway, there's my two cents.