Called to him (προσκαλεσαμενος). Indirect middle voice aorist participle. It may even be Peter's "little child" (παιδιον) as it was probably in Peter's house (Mark 9:33).

Set him (εστησεν). Transitive first aorist active indicative, not intransitive second aorist, εστη.

In the midst of them (εν μεσω αυτων). Luke adds (Luke 9:47) "by his side" (παρ' εαυτω). Both are true.