I would say light colors (best: Blue, Pink, white, green) and possibly some poke dots or design that contrast well with the wall color. If you where to do this, find some matching furniture (doesn't have to be exact, but it would just make the room look nicer) but remember this is only a suggestion.
Well, it depends on the kid really...they could be tomboys or more girly. My cousins are around that age. At my school, our Grade (12) also used to mentor the 2nd Grade kids, in Australia 2nd Grade = 7 years old. And some of the girls were into Tinkerbell or just fairies in general, Frozen, unicorns, mermaids, Minions (from Despicable Me)…that's all I can think of right now
It depends on the kid. My sister used to have pink and lavender walls (half and half, horizontally) with two different themes. First a princess theme, and then a butterfly theme. It depends on the kid, though. Some kids like different things.
my daughter currently wants tor decorate her room with an aquatic/marine theme. why not ask the children what they want, at seven they likely already have colour and theme preferences - and they are the ones who have to use and sleep in the room.
Each child is different, but usually colors like pink or lavender are good. You can pick a theme they like for example Frozen or Dora (whatever they happen to like really).