Welcome to the forum, Tdow!
What kinds of titles are you looking for?
It's ok to use public domain music to make arrangements from, but it is illegal to use copyright materials and make your own arrangements. You would need permission to legally arrange from the publisher.
Music that is currently popular is often not usually in print at the time it's movie or top 40 charts is debuting. Popular music is best bought as sheet music and there are lots of simplified arrangements and fake book music available.
Several of the method books have an all popular book music book available.
Disney in particular has different levels of fun songs for kids.
There are also some free music sites where the composer has shared their own original contemporary music compositions.
Take a look at "sheetmusicplus" which is here on the left column of this post to get an idea of what is available at retail.
The fee we pay for published music keeps the composer, the publisher, the retailer in business.
Betty Patnude
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Piano Teacher - Member MTNA/WSMTA