The device often used is commonly referred to as a nail. I'll spell that for you - N - A - I - L.
The aforementioned nail is typically made a steel wire drawn out straight. A clever machine puts a pointy bit on one end of the wire. Different lengths of nail are made, but for your purposes a length of about 20mm should be adequate. Various sorts of top on the nail are available, from a 'flat cap' to a nail for very little at the top - handily known as a 'lost head nail' - for obvious reasons.
The normal attachment device for a nail is known as a hammer. This instrument often has a wooden handle with a shaped lump of steel attached to the top. The idea is that the operative strikes the nail with the hammer (on the head, not the pointy bit, you understand - forcing the latter into the wall.
In this manner a reliable and permanent attachment device for your wife's moulds can be assured.