As a former Catholic, I wouldn't ask someone what they are giving up for Lent, but most people will be comfortable volunteering, and won't be offended if you inadvertantly offer what they've given up.
Once you know, however, I'd definitely advise holding back until Lent is over, because as others have said, it's harder for the kids to stick with their resolve than adults. You might ask Dad if he minds if you keep a box for the kids which you will turn over to him or them after Lent, or suggest giving the goodies directly to him to dispense once the season is over.
I used to bring donuts to a morning study session at law school, until a classmate gave them up for Lent. (Gee, no wonder I gained so much weight in law school!) So we switched to muffins and bagels for a few weeks so she could have breakfast too.