Are there any suggestions on how best to spend 4-5 days in this lake region in June? There is a 4-year old in the group. Should we concentrate on one lake only? Is there anything we should not miss? Thank you for any pointers.
If you want to visit both then I suggest you stay in Menaggio. It’s a very short ferry ride across to both Varenna and Bellagio, and only 45 min drive to Lugano (lake Lugano actually starts just minutes from Menaggio). There is also bus service which connects Menaggio to Lugano and back multiple times a day. I would visit Lugano and its lake on a day trip while staying in Menaggio because prices of hotels (and everything else) in Switzerland are double.
This a hotel in Menaggio where I stayed.
https://www.hotelgarnicorona.com/en
You could consider whether the Swiss Miniatur village in Melide, near Lugano, is appropriate for the 4 year old.