Jesus had openly started his earthly ministry; he sent his disciples out openly; they openly spoke in public about the kingdom of God. They were not doing anything secretly, or trying to do things surreptitiously. They also performed miracles openly, as part of the testimony that the kingdom of God was actually in their midst, with power. All the people could hear and see that which they had long hoped for.
So, what caused Jesus to exclaim in joy about God the Father having "hidden these things from the wise and understanding and revealed them to little children"? What things were hidden from some, but not from others? Well, the previous verses showed Jesus' authority enabling the disciples to triumph over all the powers of darkness that were actively trying to deflect attention from Christ and who he was. This is seen in all the gospel accounts: the Jewish people were being terribly afflicted by demonic powers. When the seventy returned to Jesus with myriad accounts of demonic powers being cast out and made subject to the authority of Jesus' name, he said,
"I beheld Satan as lightning fall from heaven" Luke 10:18 A.V.
Nobody else saw that. Only Jesus. Yet the seventy also saw the 'fall' of Satan's demons, simply whenever they commanded them to leave, using only the authority of Jesus' name. When Jesus followed that up with stating that vision, they would not doubt it for a second. Quite the opposite, they would 'see' (understand) that such an invisible thing had happened because they had witnessed the result of Jesus' power over Satan and the demons.
The people who had been miraculously cured of demon-possession would also 'see' (understand) this manifestation of the power of the kingdom of God. They would likely be poor people in the main, those despised as cursed by their religious leaders for not knowing the law (John 7:48-49). The kingdom of God remained hidden from those leaders. They had heard the same Kingdom message and seen the same Kingdom miracles, but their ears and eyes remained closed. God had hidden that from such proud men who refused to turn to Christ as the Lord's Anointed, the King of God's Kingdom.
That is why Jesus went on to say that no man can come to him unless the Father draws him (John 6:44 & 65). Jesus also said that the words he speaks are spirit and life (vs. 63)
"...but there are some of you that believe not. For Jesus knew from
the beginning who they were that believed not, and who should betray
him." John 6:64 A.V.
That gives the answer: the presence of Christ and the proclamation of the Kingdom all happened openly, confirmed by miracles of casting out demons, and healings, but God would only open the ears and the eyes of some so that they could 'see' with understanding the significance of all that, whilst others who heard and saw the same would not understand, for they would not believe.