My advice is to address the parcel to the child and don't deliver it via the mother. A post bag or brown paper wrapper over the xmas paper and in the letterbox would do. Put a card in, you don't have to sign it saying something like this is a mystery gift from someone, hope you enjoy it.

 

Many 11 year old boys are into smelly stuff.

As in Lynx body spray, hair product etc. That is what I have got my 12,11 and almost 10 year old nephews - on the advice of sis and my MIL. As sis put it (her boys were in ear shot of the phone) the younger one likes that sort of thing, the older one needs it. It fitted into my tight gift budget.