Here is what Hornady posts about their 230g XTP bullet. Reading down the page one sees they say it has a usable velocity range of 600-1650 fps. It seems to me this bullet would be ideal for grand daughter.

Testing this past summer showed that most bullets won't work at low speeds; they don't expand. This here bullet would continue to expand at speeds, and below, at which the 240g, or 250g bullets fail to work. Here is a link to a test of the 250g XTP pushed by 50g Blackhorn at 25 yard. The bullet was traveling too slow to expand. The 250g XTP--80g bullet was also tested when pushed by 80g of Blackhorn at 25 yard; it worked great.

Another bullet that works at low speeds is the Lehigh Defense Controlled Fracturing bullet. This is THE bullet one can count on to work at all speeds.