First: I am not an expert here. Most of what I have I got from Fr. Pinto or learned from my Mom and other moms. All these are good ways for grown ups to pray, too! If you have ideas, I'd love to hear them! I will do 2 methods per post.
We usually pray formally several times a day: when we get up, have meals, go to bed , and before we start our home school day. We use a lot of rote (memorized) prayers this way: Prayer before Meals, Our Father, Hail Mary, Morning Offering, Glory Be, Act of Contrition, and each child has their own "special" prayer which corresponds to how we dedicated them to God when we discovered their existence. Rote prayers are great! The kids memorize them with basic understanding and their understanding should deepen and grow throughout their lives.
But. Rote prayer gets boring for kids if it's the only way they pray!
Empty Chair Prayer
Use 2 chairs. Sit in one and invite Jesus to sit in the other. Really use your imagination and faith to picture Jesus in the chair. Just talk to Him. Little kids often need to do this out loud, so it's good to do this in a quiet room if you have one. For adults, this is great to do while you are driving if you have no one in the passenger seat, put Our Lord there.
Often kids (and grown ups) feel bored or tired with prayer because they can't physically hear or see God speaking. They have to learn to hear Him with their hearts. The imagination is part of what God uses. Not that everything in our imagination is from God, but God does use it to speak to us in prayer.
Sometimes my kids say they don't know what to say to Jesus. I often suggest they tell Jesus a knock knock joke or anything else they would say to their friend. The point is that Jesus is real and really present. I like to tell my friends jokes and laugh with them, it's part of who I am, so I like to tell Jesus funny things. Realistically, A) you're not going to find a better friend than Jesus, and B) if you can't be yourself with God, who already knows what you are like and loves you anyway, where are you going to go?