We can do both.
As soon as we wake up in the morning, we should make a “morning offering,” a prayer in which we tell Jesus that everything we do that day will be for Him.
As the day goes on, whenever we think of it, we can offer Jesus whatever we are doing. For example, “I am doing the dishes for You, Jesus, and for the souls in Purgatory.”
When we go to Mass, we should make a special intention for which we offer the Mass (for our families, for someone we know who is sick or suffering, for sinners that they may repent of their sins, etc.). When the priest offers the gifts of bread and wine at the Offertory of the Mass, we can mentally place the offering of ourselves on the paten with the host: “Heavenly Father, I offer You this bread and wine, along with myself and all I do and all I suffer, for (name your intention).”
We can do the same at the Consecration, when our gifts have become the Body and Blood of Christ: “Eternal Father, I offer You the Body, Blood, Soul, and Divinity of Your dearly beloved Son, our Lord Jesus Christ, along with myself and all I do, for (name your intention).”
You may want to simplify the words to fit a third-grader’s understanding.