Yes — the sentence is correct and sounds nice for a child to read.
Here is a simple step-by-step look:
- In spring — tells when this happens.
- birds — tells who the sentence is about.
- come back — tells what they do (they return).
- from their winter homes — tells where they come from.
- to build nests — tells why they come back (their purpose). The sentence ends with a period.
Small note: The comma after the time phrase (In spring,) is fine but not always needed. Both of these are correct:
- In spring, birds come back from their winter homes to build nests.
- In spring birds come back from their winter homes to build nests.
One easy alternative word is return instead of come back: In the spring, birds return from their winter homes to build nests. Both versions are correct. Great sentence!