We had problems with our son who is now 3. Does the 3 1/2 year old wear pull up or diapers? The first thing I did with my son was take them away so he could feel when he is wet. Any major crisis in their life will delay potty training. We moved 4 times while potty training my son and I think thats why it took so long, but once things were calm and I worked with him consistantly he was potty trained pee wise in about a month and only recentlty is pooping on the potty. So this is what all I did:
I made him wear underwear and everytime he had an accident I raced him to the potty and calmly told him yuou are suppose to go in the potty. I did this everytime.
I set the timer for every hour and just took him. If it had been a while since he went without any accidents I took him more often .
In the beginning we rewarded him with a cookie cause he loves cookies, but once he was going more than a few times a day we did a potty chart. We went to the store and let him pick stickers out and everytime he went he got put a ticker on this train chart I found on the Internet and printed. he loved being able to pick out stickers and place them on the chart. Now when the chart is all filled up I take him to the store and he gets to pic out a small present like $5 and under is what I did. You can even go to the dollars store. This potty chart really helped. Once he was going alot he was not really interested in it anymore.
It's great that you are there for the kids. I have two and they are 16 mos apart and now its time I start my daughter when I am barely done with my son! So I know that feeling! Good Luck.
A.