v.40 Receiving a disciple is like receiving Jesus because the disciple bears Christ’s message and authority (much like an ambassador). And receiving Jesus is like receiving the Father. See Mk 9:37, Lk 9:48, Jn 12:44-45 for similar statements.
v.41 A prophet is someone who speaks for God. If you receive a prophet and his message you will share in the prophets reward.
v.42 There is also a reward for receiving children because of their commitment to Christ. See for example Mt 18:10.