243
I have shot thousands of 243 and I have had good success with the 90 gn Speer flatbase bullet and IMR4350. This combo has shot well in 4 different 243's I've had over the years and the bullets perform very well on lighter game. I would recommend Nosler Parts for deer sized game but none of my 243's shot them well enough for my liking. The 90 speer worked very well as a second choice though.
Great choice for your daughter, my son at 11 took 6 antelopes and a warthog in Zim with the 243, worked flawlessly. He had 5 -1 shot kills and a little problem with his Impala, which was poor placement not cartridge or bullet problems. The 90 gn speer gave "through and through" performance on all animals up to small deer size and on shoulder shot warthog. PH was skeptical of the 243 at first but was a believer after a couple of days.