A teaspoonful of ordinary table salt kills dandelions inc. the root (may need more if it is a big dandelion) - is cheap, and is not dangerous for children and animals.
It can be dispensed as needed straight from the salt pot for spot weeding on the lawn.
Can also be used to eradicate weeds between paving stones.
Added bonus - lethal to slugs and snails.
(I have been gardening today :))