L.W.
Wow, there are so many factors that can play in to this question. Teenagers are able to watch younger children in the state of Texas. It depends on their ages, but the older they are the longer they can be left to be watched by the oldest. You can look this up on one of the official websites, sorry I don't have the link as I type this. However, being very familiar with your situation, I would not leave siblings by different parents alone unless their relationship is extrodinarily different than most half-siblings. You are right, many things could happen and why put them in a situation where the other(s) might feel the blame or to be blamed for something happening. Regarding this factor, if there is somewhere the kids can go to alleviate the time they spend alone with each other, offer it.
Regarding your ex... I don't know his situation with child support. But... if he is paying on all three, that is a big financial chunk taken out of his check. Many people do not realize this. Yes, it is their responsibility to do so, but this cuts back on the things he can do with them while they are with him. Please don't take this as a pity party on people that have to pay child support. I am married to a wonderful man that paid child support on 3 from day 1, until the 2 were old enough not to be paid for and the youngest lives with us and we have 2 little ones of our own. For years, we went without because it is such a big percentage taken out. Prior to my husband and I meeting, he had to live with his parents in order to take care of himself AND pay child support. We both have worked full time with decent salaries, and still had a hard time. The biggest thing you need to be aware of, many men in this situation can suffer from depression without you even knowing it. It sounds like at this time, anyone willing to help him out would be beneficial. The state of Texas does not care if a person (man or woman) is jobless, they have to catch up later with missed support. My husband's ex is faced with having to pay us child support now since the youngest lives with us. Whew, is it a different story.... she cried that she couldn't get by if she had to pay us. When we hit rock bottom financially, she went ahead and took us back to court for an increase since the 3 year review period came up. We nearly lost our house and cars, yet her support went up. God intervened and I was able to save my husband. I know this is lengthy, but there are so many factors that affect couples and ex-couples that we all don't think about.
It sounds like your relationships are not bitter. I hope this is accurate. You are all in the same boat together and one of the captains is in trouble. The care and safety of the children should be the most important thing. If you or the other ex can help him out, that is what I would suggest. Remember, the children will act and react to how you all handle this. I hope this helps!
I am a married mother of 3 step-children ages 20, 19, 15 and two of our own, ages 6 and 3 1/2.