R Steves always re commends Hydra because it's part of many of his trips, or else is an easy add-on for a day-trip from Pireaus. If you are traveling with that size of group, on an island that bans cars, you would be limited to the waterfront area pretty much. They also push Santorini... RS recently did a video based on a cruise trip that stops at Santorini, Rhodes & 2 other places... that also may account for the strong promotion of Santorini. If you go to Santorini, you can get by using the busses, but the hotel costs are so high, that is a consideration (BTW, for best advice, you absolutely need to give key facts, such as the budget limit you have for hotel per night for 6 + baby -- otherwise we'd waste time & effort providing suggestions based on guesswork).
A group of 6 + toddler would require a rental of TWO cars, so you'd want to think hard about island choices ... perhaps it would be good to look at islands that don't require a car as much. Are the 6 adults all the same age, or are there 2 couples who are youngish ( 30s - 40s) plus a retirement-age couple (which may be footing the bill)?? Differences in ages, & interests, may affect where you want to go & what area of an island you want to stay in?
Have you considered the cost/time required to go to islands? A ferry trip to Santorini takes 8-9 hours (fare about €55-60), a ferry trip to Naxos takes 5+ hours (Fare about €38-40)... are you prepared to pay for a domestic flight for 6 at least 1 way?? (this will vary from €60 - €100+ for Naxos -- and €70 - 250 for Santorin) -- flight for Naxos is 45, flight for Santorini is 50.
The "vets" on this forum can give some very concrete & specific suggestions for enjoyable & do-able choices IF you'll provide the above info ... ASAP, because the best values for late May already are being booked, in planes & lodgings.