Do you think I'm hurting my workplace?

So I've worked at my current job since 2016. I then got pregnant and went on maternity leave for a year before returning in 2018. I worked full time and worked up to 7 months pregnant. I'm now only able to work the 4 hour morning shifts on sunday (I watch my son during the week) and have been doing this since I came back in 2018. Sometimes I wonder why they keep me if they can get one of the other workers to do the 4 hours and it makes me nervous. My boss always asks me when I'm coming back to work during the week and its awkward because I'd have to wait for my son to be in school (hes two).

Do you think I'm a burden? Do you think they'll fire me eventually? I work at a doggy daycare by the way. My job is to take care of all of the boarding dogs on sunday mornings along with other small cleaning tasks. I work alone every sunday unless its busy. Thanks :)