Be aware that if you buy female livebearers (guppies, mollies, platies, swordtails, etc) they may have already been kept with a male while sexually mature. If they were insemenated they can drop that batch of fry and continue dropping fry for upwards of 6 months, because they store sperm for that 6 month period. The stored sperm is used, by the female, to reinsemenate itself.
Safer to go with all male, but the livebearers tend to eat their fry. Females get a round belly and tend to not have as much color on the body, in the lower quality strains like the chain stores have, as the males do.
That is nice of you to get your nieces something nice, and be willing to do it responsibly as well!
Hope they enjoy it.
