Are they refusing it, or just holding it up?
The Canadian Border Agency Site is a bit confusing, but if what you're sending would mostly fall into the "gift" category, the declared value shouldn't exceed $60, which is (currently) about £37. In the case of second-hand baby clothes, I wouldn't have put any monetary value on it at all.
Anyway,
this section of their website may (or may not) be of some help for future mailings.
EDITED: 9 Nov 2012 14:33 by JELLS