Closing the windows in not helping the lead problem. It may make no
difference at all, but OTOH, opening the windows will disperse any lead in
the air making it safer. Is your kid eating the windows? Simple contact is
not a big deal.

Actually, the primary danger from lead paint in windows (assuming the
paint isn't flaking/peeling, which ours aren't) is in opening/closing
them. Each cycle produces lead dust (due to surface rubbing) which
then gets distributed inside the house. Keeping the windows closed
eliminates the biggest threat from them.

If a kid is actually chewing on them, that is another story....