I did have a question when did you start potty training the 4 and 5 year old? Has it been a long time or short time?
If it has been a longer time then they know better and even though its related to stress/there dad leaving for awhile they need to use the potty especially at 4 and 5. You need to let them know that it is not okay to ahve accidents just bc they don't want to go. Unforteniatly it comes down to punishing them for not using the potty. If they understand and just refuse to use the potty start putting them in the corner and letting them know that you don't like there behavior. They are just too old to have accidents bc they refuse to use the potty.