Do you remember when summer school was a bad thing? Well not anymore! The team you manage is faced with the need to learn continuously. And it takes more than just an order from you to motivate them. They are probably doing the work of one-and-a-half people already, so adding in more training could become a distraction. But it doesn't have to.
First, set a specific time period for this year's "Summer School" like Memorial Day to Labor Day or if you prefer Memorial Day until the 4th of July. Make the dates specific. Second, select a clear and single topic. This should be any topic that is important to all of the team: i.e. communication, effective emails, problem solving, or it can be specific to your industry.
Second, select material online or that you will distribute that will be the "textbook" for what the team will be studying. Don't do too much...just some web resources or chapters from a book. Make it doable.
Third, set aside time each week for the team to huddle in-person or an online chat to discuss what they are learning. Be as creative as you can. It can be a weekly lunch and learn or coffee and a chat. It doesn't have to be more than 30 minutes. Just make sure you do it every week. You can ask a different person each week to lead the discussion.
Summer school is a good thing. Your team must keep growing forward. Now, it's up to you to make it happen.