Hi Scarlet. It sounds like your daughter may have some natural talents there.
With the little boy who is being bad....you need to tell her it's OK for her to tell him to go away. As with any other "child" she might interact with, she has the right to tell him she doesn't want to "play" anymore. If she is actually a medium, which she may well be, she needs to learn early that this is something she can and should do with any spirit/ghost she comes across that disturbs her or who doesn't act nice. The sooner she is aware and understands she's allowed to have some power over this, the better control she will have on who or what comes into her psychic world.
For a child this young, it's difficult to empower them at a level they understand unless you use the simple "good vs. bad" approach. If he's being a "bad boy" she needs to tell him to leave. If someone is being "good" and she doesn't mind hearing their stories, she can allow them to stay. Make it as simple as possible for her at this age.
You can also "ward" her room with holy water or medals, as laid out in the link Lynne gave you. Another thing might be to make her a very nice little necklace of a St. Benedict's medal that will hep protect her from the "bad" stuff out there.
You, as her mother and protector, also have the power to send away this bad boy spirit. You need to talk directly to it when she tells you it is around and state it is no longer welcome in your daughter's life nor is it allowed to touch, invade her dreams, or bother anyone else in the household. Reclaim your space and your daughter and do it with authority.