Your intentions are pure and of the Lord. It is never a contradiction to teach your children more about the beauty and gift of our Catholic faith. Using materials that are truly Catholic in nature will ensure that your instructions will be peaceful and true. I think that there is great wisdom in making a purposeful effort to formally teach the faith to your little boys.
However you are correct in using caution, so as not to overload them with too much tutorial and risk their turning away from the lessons. I would suggest that you purchase one of our religion texts suitable for their age and make your formal instruction in religion the beginning of your summer homeschool. Pick a time of day and a day that will afford you the least interruption. I think that it is always better to teach consistently when teaching little ones. One hour, one day a week should be a good start. As they ask for more, expand your time. It is very important that you stick to the schedule that you set. In this way you show the boys that this teaching is a priority. For example to cancel your time together for outings etc. sends a message that outings are more important. This is why it is important that you decide upon a time that you feel will best suit your family situation.
I admire your dedication and zeal for parenting. It is a holy witness to strive to bring our children closer to the Catholic faith. Parents are the first and therefore most important teachers that our children have. Jesus has given your children to you from His great love for the children. Go forward into this important work.
Let us pray together a Hail Mary. Dear Mother of our Lord Jesus, teach us tenderly to follow in your witness. You were a young mother who taught the child Jesus. Show us the way to reach the hearts of our children so that they will grow in their love of Jesus. Amen.
Sending out a prayer,