Bluntschli has spent fifteen years as a soldier, in "barracks and battles". He has lived a life of adventure, which for him has been a serious business, with his very life frequently at risk.
He suggests that Raina, being much younger than he is, has "her imagination full of fairy princes and noble natures and cavalry charges and goodness knows what" -- that while to him a soldier's life is a real, dangerous, muddy, hungry, unglamorous affair, to her it is the stuff of exciting but unrealistic fiction.
A bit of context for the specifics of the quoted text: Raina and Bluntschli met much earlier in the play. He was hiding from enemy soldiers, and she kept him hidden and gave him chocolate creams to eat. (Hence "chocolate creams and hide and seek" -- though of course the way he puts it is also meant to put you in mind of children's food and children's games.) He shocked ber by being prosaic and matter-of-fact about soldiering, contrary to her heroic imagination of that profession.
So: He thinks (or at least professes to think) that her view is childish and foolish, while his is grown up and realistic; that when they met before, he was trying to save himself from an unpleasant premature death, while she was just having fun; that all this points up the difference in age and maturity between them. And that, as she is only a child, he can't take seriously any interest she may have shown in him. How seriously he means this isn't made explicit: maybe he means every word, or maybe he's just being provocative.
(It then turns out that she is 23 not 17, and things turn out as one might expect.)