I like parent/tot classes depending on the "culture" of the class, because less carrying kids around for me...I'm lazy maybe? It is kind of annoying when a kid won't do something, runs away, etc, and the parent does nothing but you aren't sure whether they expect you to do something or if you'd be stepping on toes. Awwwkward.
I usually just redirect and keep talking to them (as well as change up the spotting station). When you can spread a circuit out on a big clear floor and play kid friendly music, beautiful things can happen. I've had a few weekend off time PS classes with an empty gym and it's such a wonderful experience. Unfortunately you're usually crammed in a corner somewhere with a million distractions for them to climb on. Makes me want to cry sometimes...anyway usually doing silly things helps and just repeating yourself.
Also you should keep in mind appropriate expectations of course, but you kind of have to gauge the extent to which they're testing the situation. If the expectation is basically reasonable (for the individual PSer...this is where it gets hard because a couple months at these ages can yield some disparate abilities) then there's some point where you have to draw that line in the sand and never let them cross it, or else the entire level of the class goes down and it's always an uphill battle to regain "control" as it were. I'm a pretty easygoing person and to some extent that's my biggest problem working with little kids. Also I have the attention span of an ice cube myself and I'm usually the one watching the clock ready to bolt to the next thing as soon as I can peel them off, so...yeah. But anyway the thing there is, if you're bored or acting bored they pick up on it and get antsy too. So I try to do silly voices, always talking to as many of them as I can, praising them with over the top and new adjectives, dancing, etc.
Anyway I usually repeat rules before anything happens while they are sitting...no running is a big thing for me in circuits. Sometimes the little kids forget but you can't let it go when they do - every time - go stop them and remind them no running from station to station (only running if the station involves running). Or whatever that particular rule is. Otherwise things can go downhill really fast when they start to get comfortable with you. I have a lot of ways I say things "seriously" without being too loud. I catch them, get down on their level, make eye contact, and go "ahhhhh!!!! I think I saw running! We can NOT run in the gym because there are a lot of people. I want to see you walk slowly like an elephant. Oh yay you can!"
Also sometimes giving them something to chant or a noise to make while they do a station makes it a little more interesting. Depending on the age. Always have a little colored mat or carpet or dot or chalk mark even if the station doesn't need it...find creative ways to put them so they can see exactly where their hands/feet/bottom goes on a bigger mat or block...this makes such a big difference for me. It defines the space more for them, gives them a visual reference so they can recall the directions faster, and they get way less likely to skip stations or just half run through them. If they whine about not wanting to do a station or wanting to stop, I say, "I bet you are really good at that one! You are? Show me!" They always want to show.
Yeah so I have no life...apparently.