May and September are the two "shoulder season" months that regularly come up when families ask us for advice — both sit outside the peak summer crush, but they feel quite different on the ground. Here's how they actually compare when you're travelling with a baby or toddler.
Weather and Sea Temperature
May is the gentler of the two. Air can still feel cool for sunbathing on certain days, and the sea itself is still a bit chilly — so if your toddler is used to warm bathwater, expect some hesitation on the first paddle. Daytime temperatures are comfortable rather than hot, which is actually a relief with young kids, since you're not fighting the midday heat to keep them cool and rested.
September, by contrast, is still very much summer. Average highs sit around 28°C with evenings staying warm near 19°C daily temperatures generally range between 22–27°C with the sea still warm enough for comfortable swimming. In practice, this means September beach days feel just as "proper summer" as July or August, but without the extreme heat spikes.
For babies especially, this matters: intense heat is genuinely hard to manage with a baby who can't regulate their own temperature well. September gives you summer swimming conditions with noticeably less risk of overheating during the day.
Crowds and Atmosphere
This is where the two months diverge most for families:
- May is quiet. Beaches, tavernas, and attractions are far less busy, which means more space to spread out with a stroller, shorter waits, and a generally slower pace — good news if you're travelling with a baby who naps on a schedule you need to protect.
- September, especially the first half, still carries peak-season energy. Family bookings often overlap with the tail end of the summer holidays, so expect more company at popular beaches, though it does gradually quieten as the month goes on.
If a calm, unhurried trip is your priority, May wins easily. If you want livelier resort atmosphere, September delivers that while still avoiding the peak-August squeeze.
Practical Considerations for Families
Swimming with a baby or toddler
- May: pack a wetsuit-style swim layer if your little one feels the cold easily — the sea takes longer to warm up than the air.
- September: the sea has had all summer to warm through, so it's a more forgiving month for babies who are sensitive to cold water.
Sun safety
- May's milder sun is easier to manage, though UV can still catch you off guard on a clear day — sunscreen and shade still matter.
- September sun is stronger and closer to peak-summer intensity, so treat it with the same caution you would in July: hats, shade, and an early or late beach window.
Availability and pricing
Both months tend to be quieter and often better value than the height of summer, though September (particularly early September) can still command summer-level pricing at popular family resorts, since it overlaps with school holidays in some countries. May is generally the better month for availability and pricing if your dates are flexible.
What's open
Kos is a well-established family destination, so restaurants, rentals, and attractions are running in both months — but a handful of smaller, seasonal beach bars and businesses may not be in full swing yet in early May. Worth checking specific spots in advance if there's somewhere you have your heart set on.
Our Take
If your priority is a calm, unhurried holiday with a baby and you don't mind the sea being a little cooler, May is hard to beat — quieter beaches, easier pace, and comfortable (not exhausting) heat.
If you want the full "warm sea, sunny beach days" experience with your toddler but without the peak-August crowds and heat extremes, September is the sweet spot.
Either way, the packing and gear logic is the same: leave the bulky items at home and let them be waiting for you.
Whichever month you choose, we'll have your stroller, car seat, and other essentials ready at your accommodation when you land — browse our rental gear
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