Called to him (proskalesamenos). Indirect middle voice aorist participle. It may even be Peter’s “little child” (paidion) as it was probably in Peter’s house (Mar 9:33).
Set him (estēsen). Transitive first aorist active indicative, not intransitive second aorist, estē.
In the midst of them (en mesōi autōn). Luke adds (Luk 9:47) “by his side” (par' heautōi). Both are true.