I suppose it's possible that the slogan was originally conceived without any dodgy intent (in a spirit of bob-a-job week and the Secret Seven). But I rather doubt it and agree not really appropriate to dress a child in as it will certainly be read with dodgy connotations.
Could you cut the sleeves down to short-sleeved so it just has "YOUNG" (or "EAGER", I suppose, depending on which way the writing goes)?