A partial dialog, where one unsympathetic question leads to another:

Child: Why are these dirty needles left lying around?

Parent: The same reasons dirty liquor bottles and cigarette butts are left lying around. The people who enjoy these products hate always being judged harshly, and are sometimes afraid of being arrested, so they're in a big hurry to hide or get rid of the evidence. Some of them litter because messy things don't bother them right after they've had drugs, (but they may feel sorry about the mess later). But some of them litter as a petty revenge on the cleaner people who blame them for being messy.

C: Why can't they just do all that at home?

P: Not all of them even have a home. And some who have homes are yelled at or punished if they bring the products home.

C: Why can't they go to a hospital?

P: Because doctors are afraid of losing their jobs if they gave them as much drugs as they'd ask for. Also, not everybody can afford to pay the hospital's prices.

C: [Now thinking of an easy solution] Why can't they all go to jail until they promise to never use drugs or litter again?

P: Well there are too many people who use drugs, and not enough jail cells, and jails can be very expensive; so that the city might not be able to afford a Zoo or a playground because it spent all its tax money on jails. Also often people in jail promise not to do things, but then leave and go off and do the same things anyway.

C: [Growing more vexed]: Why can't we just take away all the bad drugs? So nobody can use them.

P: It's been tried, and police work very hard at it, but it's more difficult than it sounds. We can underestimate the determination, cleverness, and forcefulness of people that enjoy and supply illegal drugs. They work hard too, and some of them become very rich, richer than police, and pay jealous and unhappy policemen to help them. Also most of the drugs have good uses as well, and many people would suffer and die without them.

A partial dialog, where one unsympathetic question leads to another:

Child: Why are these dirty needles left lying around?

Parent: The same reasons dirty liquor bottles and cigarette butts are left lying around. The people who enjoy these products hate always being judged harshly, and are sometimes afraid of being arrested, so they're in a big hurry to hide or get rid of the evidence. Some of them litter because messy things don't bother them right after they've had drugs, (but they may feel sorry about the mess later). But some of them litter as a petty revenge on the cleaner people who blame them for being messy.

C: Why can't they just do all that at home?

P: Not all of them even have a home. And some who have homes are yelled at or punished if they bring the products home.

C: Why can't they go to a hospital?

P: Because doctors are afraid of losing their jobs if they gave them as much drugs as they'd ask for. Also, not everybody can afford to pay the hospital's prices.

C: [Now thinking of an easy solution] Why can't they all go to jail until they promise to never use drugs or litter again?

P: Well there are too many people who use drugs, and not enough jail cells, and jails can be very expensive; so that the city might not be able to afford a Zoo or a playground because it spent all its tax money on jails. Also often people in jail promise not to do things, but then leave and go off and do the same things anyway.

C: [Growing more vexed]: Why can't we just take away all the bad drugs? So nobody can use them.

P: It's been tried, and police work very hard at it, but it's more difficult than it sounds. It's so easy to underestimate the determination, cleverness, and forcefulness of people that enjoy and supply illegal drugs. They work hard too, and some of them become very rich, richer than police, and pay jealous and unhappy policemen to help them. Also most of the drugs have good uses as well, and many people would suffer and die without them...

Note: The dialog could be made more sympathetic, (i.e. why folks use and why some oppose that), but it would be longer, so this answer takes the easier road of posing conundrums in hopes of slowing down the rate of such questions so the parent is not exhausted.