Warmer weather on Christmas

Hi, I'm planning to bring my 2 years old toddler to visit Japan on Christmas. I'm looking for a warmer weather (maybe around 10C - 20C) place in Japan for it won't be too cold for my toddler. Is there any place to recommend? I have been visited Tokyo, Osaka and Kyoto before during Christmas and I found it is too cold for my toddler.

Thanks for the advise.

Lower elevations of Southern Kyushu and Okinawa are usually above 10 degrees:
https://www.japan-guide.com/list/e1108.html
https://www.japan-guide.com/list/e1247.html

Curious, then and I have checked with https://www.benricho.org/weather_ratio/. Only up to a few degrees warmer on the main islands. Warmest in Honshu is Shizuoka, in Shikoku Kochi, and in Kyushu Miyazaki. Only Okinawa satisfies your needs. However, in Okinawa, it is not a nice weather.

Kyoto 12/25 60 10 13 17 1.3 6.0 10.4 2.2
Osaka 12/25 70 10 10 10 1.1 7.3 10.9 3.8
Tokyo 12/25 76.7 10.0 6.7 6.7 1.4 7.3 11.2 3.6

Matsuyama 12/25 57 10 23 10 1.3 7.2 11.3 3.3
Wakayama 12/25 70 3 17 10 1.3 7.3 11.3 3.6
Tokushima 12/25 70 10 10 10 1.3 7.3 11.3 3.7
Fukuoka 12/25 43.3 10.0 26.7 20.0 1.7 7.8 11.4 4.4
Hamamatsu 12/25 70.0 10.0 10.0 10.0 1.5 7.3 11.6 3.6
Kumamoto 12/25 46.7 13.3 26.7 13.3 1.5 6.6 11.7 1.8
Oita 12/25 60.0 13.3 16.7 10.0 0.9 7.2 11.8 2.9
Omaezaki 12/25 90.0 0.0 10.0 0.0 2.0 8.0 11.9 4.2
Nagasaki 12/25 46.7 10.0 23.3 20.0 1.7 8.3 11.9 4.8
Atami 12/25 64.3 17.9 14.3 3.6 1.6 8.5 11.9 5.4
Oshima 12/25 69.6 8.7 13.0 8.7 3.3 8.7 12.2 4.9
Irozaki 12/25 70.8 8.3 20.8 0.0 2.0 9.7 12.3 7.2
Owase 12/25 73.3 3.3 13.3 10.0 2.9 7.4 12.8 2.5
Shizuoka 12/25 73.3 10.0 13.3 3.3 1.9 7.8 12.8 2.8
Kochi 12/25 73 3 13 10 1.6 7.3 13.3 2.5
Kagoshima 12/25 60.0 16.7 20.0 3.3 2.0 9.6 14.0 5.3
Miyazaki 12/25 83.3 3.3 13.3 0.0 1.5 8.6 14.1 3.5

Naha 12/25 43.3 26.7 30.0 0.0 3.7 17.7 20.3 15.4
Miyako Is 12/25 43.3 20.0 36.7 0.0 5.0 18.8 21.1 16.8
Minami Daito Is 12/25 56.7 13.3 30.0 0.0 4.5 18.6 21.5 15.4
Ishigaki Is 12/25 43.3 20.0 36.7 0.0 4.7 19.2 21.8 17.0

Okinawa Ishigaki Island in December: Warm, but not a good weather.
https://www.benricho.org/weather_ratio/47_4_12.html
Miyako Island: Similar
https://www.benricho.org/weather_ratio/47_3_12.html
South Daito Island: Sunny and warm. But hard to get there.
https://www.benricho.org/weather_ratio/47_2_12.html
Okinawa Island: Better
https://www.benricho.org/weather_ratio/47_1_12.html
Kagoshima: Sunny, but colder
https://www.benricho.org/weather_ratio/46_12.html
Miyazaki: Colder
https://www.benricho.org/weather_ratio/45_12.html

The further south you go the better/warner. But honestly. If you don't like cold weather, don't go to Japan in winter.