This is the 400th most frequent Finnish word.
varsinkin
"Varsinkin" means "especially" or "particularly" in English.
Here 'varsinkin' emphasizes or highlights the specific time period, summers, where the activity mentioned (enjoying swimming) is particularly true or relevant.
Varsinkin kesäisin pidän uimisesta.
Especially in summers, I enjoy swimming.
In this context, 'varsinkin' specifies or underlines football as the primary or notable example among all sports mentioned.
Hän pitää kaikesta urheilusta, varsinkin jalkapallosta.
He likes all sports, especially football.
Here, 'varsinkin' serves to single out or give particular emphasis to children as a notable group.