Jelly toys are saturated with plasticizers that keep them soft - and that smell really bad. Washing them off removes it from the surface temporarily - but in just a few minutes the smell is back.
Leaving it our will permit some off-gassing to occur - but the smell will linger indefinitely. When the small goes away, the toy will be much harder - think of your dashboard after a few years in the sun.
The best advice is to avoid the smelly toys all together.