The original may not make suitable reading for young children who aren't ready for adult material (I'm thinking of the sexual content; extreme violence doesn't seem to be considered a problem as a rule). Medb tends to feature as an important character in other people's stories rather than on her own, so most childrens' books based on the Ulster Cycle will probably feature her, though they may not give a very accurate picture. Lady Gregory's re-tellings of the Cuchullainn stories might be a start, though there are bound to be more recent (and easier to read) books that other people here will know about.

Searching for Medb rather than Maeve will probably get you more useful results.