Any simple grammar rules for these two to splain to the kids with?
I unagree with Fluffy.
It’s a good question. From the top of my head:
[quote]The Collins COBUILD (HaperCollins, 1995) has simple distinctions for these two prefixes. The entry under dis- says:
" Dis-" is added to some words that describe processes, qualities, or states, in order to form words describing the opposite processes, qualities, or states. For example, if you do not agree with someone, you disagree with them; if one thing is not similar to something else, it is dissimilar to it.
The entry for un- says:<
“Un" is added to the beginning of adjectives, adverbs and nouns, in order to form words that have the opposite meaning. My father was an unemployed labourer…He had sensed his mother’s unhappiness….
" Un-" is added to the beginning of the past participle of a verb, in order to form an adjective that means that the process described by the verb has not happened. The theory remains untested…Dealers across the country continue to complain about huge stocks of unsold cars.
The description of dis- above describes the case of disinterested, which means that a person is not interested, is not involved, and probably never was involved. Disinterested is often used to describe a person who has no interest or involvement in a situation and therefore does not stand to benefit from it, as in: <
Gary would make a fine arbitrator for this case; he is completely impartial and disinterested.
Uninterested, on the other hand, means that the person is indifferent or bored with the situation at hand: <
The conversion with Monique got boring very quickly – she’s completely uninterested in what we’re planning, and in anything else, for that matter.
A pair of words with the same kind of distinction as disinterested/uninterested is dissatisfied and unsatisfied. Dissatisfied carries the meaning that a person is discontented, as in “Harold is discontented with his job.” Unsatisfied means that something is lacking, unfulfilled, or is yet to happen, as in “The child’s hunger went unsatisfied for days.”
These two combinations – dis and un- – both mean “not,” or “the converse of.” Dis- combines freely with nouns verbs and adjectives: disorder, disobey, dishonest, for example. Un- combines freely with adjectives and participles: unfair, unassuming, unexpected, unclear, for example.
Sometimes there is a difference in meaning, as in disinterested/uninterested and in dissatisfied/unsatisfied. Sometimes, however, the word exists with a particular prefix just because of the derivation of the base word.
For interesting insights and expansion on negative prefixes, see Quirk et al., A Comprehensive Grammar of the English Language (Longman, 1985), pp.1540-41, and Huddleston and Pullum, The Cambridge Grammar of the English Language (Cambridge University Press, 2002), pp. 1687-88.[/quote]
Again, that’s just off the top of my head.
After seeing Irishstu posting here, I was reminded that you need some simple rule to explain to kids.
As the rule is perhaps too difficult for them to comprehend, perhaps you could just play a game instead, with words that they understand the meaning of, in which they have to remember the correct prefix.
I used to play Snap! with my students, even the older ones. One kid has a pile of different prefixes, and the other has a pile of words that can take a prefix. It really helps them quickly absorb which words can take which prefixes (be careful not to include words that might take one of the prefixes to produce a real word but one that is too advanced for your students, otherwise you’ll have a lot of explaining to do, either now, or when they’re advanced students and come to realise that they could have won that kindergarten Snap! game after all, so many years ago, with unbelieve). 
disposable / unposable
disease / unease
disco / unco
discover / uncover
or perhaps:
disaster / unaster???