nat your 13 n 14 year old are certainly mature enough where their desires should be considered by both parents as likely the court would at their age. Ultimately it isn't their decision though as that is their parents job or failing that the court, the pressure of making them decide such a big life choice at their age shouldn't rest with them and they perhaps need to know that. I know you said you are worried about going against them but I can tell you as a male who once many moons ago a 13-14 boy we should rarely be allowed to achieve our desires with parental guidance lol.I guess what I mean is do I go against my boys wishes and just agree, part of me would feel like I have let them down.
The big issue is understanding why they wouldn't want to spend 50/50 with their father and mother, this is more of a soul searching thing for you and might mean you need to send them to talk to a professional who specialises in teens going through a family break up. It is possible for example they are saying that due to wanting to protect you thinking you might not handle the time apart as well as dad would. It could be that mum is a push over and lets them do things dad doesn't. It could be they have overheard you saying things about their father and what he has said or done to you and that has shaped their opinion.
I am certainly not saying that's the case with you but there just some thoughts.
The reality is it's likely you have a low opinion of their father at the moment but ultimately he is still their dad.
I am guessing the 19 month old has the same father, if so that is a totally different scenario as it's very unlikely he will get 50/50 of him/her until they are around 3 yrs old but just as important that they bond with their father.