Change the activity to make it simpler such as a picture to colour - only give the 16 month olds one crayon at a time and watch them reminding them not to put it in their mouth as they start to do that. Switch out the colour every few minutes if they seem to want to - at least offer. Then the little ones will usually lose interest after a bit of scribbling and be ready to leave the table to go play. At which point you can then let the older ones add the extra things.

If you are having to "help" the older ones to do the craft then maybe it is time to reconsider if they are actually ready for it either. While it looks nice to send home I bet they would be just as happy with the picture they coloured - cheaper too and safer when there is a mixed group.

The 3 and 4 year olds will not be doomed for life if they don't get to glue pipecleaners on paper for a few more months. That way the 16 month olds will be old enough to participate properly. In the meantime include lots of sensory type play you can control such as putting paint in a zip bag for no mess fingerpainting - tape the bag to the table for the little ones. The older ones should be able to do playdough while you supervise the little ones nearby.