While many bowlers buy an extra plastic ball for practicing straight balls, you don't need to get both at the same time. In fact, you can go right ahead and just get your child a urethane or resin ball. Most alleys have plastic balls ready as their house balls, so your children can use them to make a straight ball or hit a spare. Sure, these balls may not be as comfortable as customized balls, but using them will teach your kid how to adapt anyway.