Ex's son called me mommy

I have custody of my ex boyfriend's 3 kids because he got arrested for doing something stupid and was looking at 5-8 years. Well turns out some other dumb shit came to light abd they are trying to tack on more charges so he could be looking at 15 years... We broke up because he got disrespectful just because I couldn't give him money in prison because I'm caring for his kids. I have his 4 year old son 10 year old daughter and 14 year old son. My mom has been a lot of help with the kids. There mom didn't want them and I didn't want them to get separated. His 4 year old has been wetting the bed so be wears pull ups and started sleeping in the bed with me because he's scared and wants his daddy. My ex hasn't called to check up on them in a week... We've all been okay though but his 4 year old recently started calling me mommy... He doesn't really know how mom because my ex took full custody of all 3 kids when the youngest was barely a week. Their mom is absolutely insane and was abusive. I love these kids so much... I just hope it won't be damaging for him to get attached to me as a mommy figure, especially when me and his dad aren't together. I don't have actual kids of my own so this mom thing is new. But I've been trying to give the kids my best with my mom's help. Is it okay the 4 year old attaches to me in this way?

Edit: The reason I took them in was so they wouldn't be handed to the state. The moment their mom said she wasn't taking them I knew I needed to. I knew I was gonna have to prepare for this to be my next be 5-8 years(possibly 15). As for adopting them... Well for that to even happen their dad would have to sign his right away since we aren't even together, let alone married. I doubt he would do that and I'm sure even though they understand dad did a stupid thing and wounded himself up there, thats there father and they wouldn't want me completing taking him away. At least the older ones wouldn't. I do still want to be in their life even after he gets out and I hope that's possible.