It is a compound sentence.
Compound sentences are formed by joining two or more independent clauses together, most often with a coordinating conjunction. In this case, "so" is the coordinating conjunction. The two independent clauses can be found by searching for the subject-verb pairings, in this case: "toy was" and "mother searched and found." Remember that in a subject-verb pairing the subject can be a compound subject, e.g. "Fred and Bob," and the verb can be a verb phrase consisting of two verbs, e.g. "searched and found"). Colons and semi-colons can also be used to join two or more independent clauses, but that's for another time.
Simple sentences consist of an independent clause (a complete thought) containing one subject-verb pairing. Example: He ran to the store.
Complex sentences consist of at least two subject-verb pairings: one or more dependent clauses (headed by a subordinating conjunction and expressing an incomplete thought) joined with an independent clause. Example: After he ran to the store, his mother called him in to help her cook.
Compound-Complex sentences consist of at least three subject-verb pairings and include two independent clauses joined by a coordinating conjunction AND a dependent clause joined to them by a subordinating conjunction. Example: After we swam in the pond, we were cold, so we went inside for a cup of hot cocoa.
Examples of coordinating conjunctions: FANBOYS. For And Nor But Or Yet So.
Examples of subordinating conjunctions: because, while, after, although, etc.
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