I think this is quite a good idea. The BBC» state that people pay more attention to children's voices than adults'. Even if that's not the case, children's voices making public announcements are likely to grab attention simply because they are unusual. In addition, it introduces some children to the railway and probably makes children already using these stations feel that they're in a more child-friendly, less forbidding and confusing, place. My only concerns would be ones that apply to all announcements: are they clearly recorded and are they all necessary or useful?
On the London Underground escalators last week the safety announcements at one station, at least, utilised a child's voice. The impact was somewhat lessened by the sound quality but it did attract my attention as intended.