if it were me (and i should say-i have a professional background in child welfare) and i was truly concerned, and the parents were unresponsive my voicing my concerns (or the type that i knew i could not personaly approach) and there was noone within the family that i felt i could share my concerns with who might be able to effectivly communicate the concerns to the parents-i would likely make a call to my local child welfare unit.
it can be done anon. if you prefer, and the intake worker will listen to your concerns and observations and determine if a referal for investigation is appropriate.
child welfare does not automaticly "swoop in" and take children (believe me, there were times when everyone involved wished they could have)-they evaluate each situation and are often able to provide parents with tools/education to provide a safe environment for the children.
i would be surprised if the kids were treated by emergency medical personel (depending on the circumstances of the meds. injestion as explained by the parents) if some type of referal has'nt already been made (med. personnel are mandatory reporters for some types of incidents involving kids no matter what the parents say is the circumstance).
don't drive yourself crazy with the concept of "there are kids in much worse situations"-you have to look at the situation you are familiar with and ask yourself "are these children getting a reasonable standard of care and supervision?". with a 2 and 3 year old the standards of supervision tend to be higher than with an older child.
these 2 sound very fortunate-expired meds can be double, triple or more in strength than non expired (and far more deadly).
your situation brings to mind two tragic circumstances i am aware of. one occured in the town we used to live in-a little girl (around 3 1/2 i think) was discovered drowned in the family pool. the media started covering the "tragic accident" and interviewing the neighbors, family...well, it becomes apparant that this little girl was often found out playing unsupervised in the neighborhood and taken back to her home by neighbors, found wandering around the neighborhood and taken back to her home by neighbors, the family starts opening up and saying "yeah, there were times when we thought the mom was making the older daughter (age 5) too responible for watching her little sister-but we were afraid to say anything because they were her kids and we figured she knew best how to raise them...". noone ever voiced their concerns to the mother or anyone who might have stepped in pointed out to the mother that a 5 year old was'nt an appropriate supervisor for herself let alone a 3 1/2 year old sibling). all i can think of is that 5 year old with a lifetime of guilt over her sister's death

there was also a case that got alot of national attention-in the midwest i think-2 kids (i think the brother was around 7 and the sister around 4) wandered off and drowned in a pond near the family home. in that case child welfare had actualy done a home visit like the day before because some family and neighbors had stepped forward and made calls about their concerns re. lack of supervision. but again, the lack of supervision had been apparant to people for a long time before someone stepped forward and said anything.
if you've seen enough to be concerned there is probably lots more you are'nt seeing-i would err on the side of being overly cautious and take steps to make sure the children are safe.