2.5yo... not much sitting round at that age... I have two kids 4 and 6 now, both can swim and are used to wearing life jackets, but hated them earlier in life... we have full netting, but now that they can swim when on board full time and now only use them in the dingy. They actually like wearing the jackets better now and often wear them on the beach as well?
Seriously though, we didn't do much sailing with kids between 12mths and 3 years because they are very demanding, 100% adult attention required, which wears someone out if for more than a few hours.
Kids 3-6yrs a 15-25kg jacket is perfect. The 10-15kg jackets are really bulky and uncomfortable for little ones.
Keep your eyes peeled grandpa!