(x + 5)² is not x² + 25. The middle term is the one that goes missing, and it goes missing for almost everybody once. These free printable worksheets show the wrong line and ask which of four answers is correct.
If they get stuck
The answer key prints on a separate page.
Squaring a bracket looks like it should distribute over the plus, and it does not — that expectation is so natural it has a nickname among teachers. Writing the bracket out twice, as (x + 5)(x + 5), is what makes the two middle terms appear, and a child who has seen where they come from stops losing them.
The four answers separate the near-misses that matter: no middle term at all, a middle term that was never doubled, a constant that was never squared, and a bracket that was doubled instead of squared. Each is a different misunderstanding and the correction for each is different.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Squaring a binomial, step by step