You could find a subject she likes to collect. My 7 year-old has an interest in collecting that waxes and wanes. He really likes coins with dragons. Fortunately there are many to choose from that can be had at a reasonable price. Maybe your niece likes animals. There are many great world coins with animals that can be bought for small money. My son uses the plastic slip-type 2x2's to store his coins and I try to make sure the coins he gets are circulated so he can handle them without doing too much damage. At age 7, being able to touch something seems to be a necessity.
My 10 year-old has a different approach. He collects Swedish coins because my family has Swedish heritage. The cool thing about Swedish coins is that you can buy a 250 year-old for under $20. For some reason kids like old coins. The big Swedish coppers seem to have added appeal as well. My son is old enough that he can use Krause's SCWC. Sometimes I'll get him a mixed lot and teach him how to identify coins based on denomination, subject matter, etc. He puts them in 2x2's and prints the country name, denomination, year and KM number on them.
My 10 year-old has a different approach. He collects Swedish coins because my family has Swedish heritage. The cool thing about Swedish coins is that you can buy a 250 year-old for under $20. For some reason kids like old coins. The big Swedish coppers seem to have added appeal as well. My son is old enough that he can use Krause's SCWC. Sometimes I'll get him a mixed lot and teach him how to identify coins based on denomination, subject matter, etc. He puts them in 2x2's and prints the country name, denomination, year and KM number on them.


















