Nice game! I thought the graphics were attractive and the music was good, as far as 8-bit style goes. The rooms were fun to figure out & the mechanics were good. I got myself killed a few times by jumping in the middle of mom's dash accidentally, but since you need to jump out of the dash later on, it's OK.
I have some of the same crticism others gave - faster walking or a run button, faster character switching, & the room before the 4th kid is too hard. I was lucky enough not to land on a checkpoint when missing jumps in the final area, but that seems like a big issue. I'd suggest making that impossible by putting a tile right above the checkpoints in that area, or just changing them in some other way, like requiring a button press to use them.
My other biggest issue was the checkpoint placement. In both of the hardest parts, the checkpoint was in a place where you have to do a little boring footwork before the actual hard challenge. If the checkpoint was directly before the hard part, it wouldn't be as annoying. There was also at least one optional item that looked dangerous to get, but the nearest checkpoint was very far away. I would've loved to try to collect all the items, but it's not worth it if you lose a significant amount of progress by trying.
I really liked a couple small things. Getting to keep the collectables after dying was really nice. I really liked the aesthetics of the collectables in general, especially with them all being unique. It made me want to try to get them all.
Having the kids be right next to a spot where you need their ability was a smart way to let you know how to use them without actually needing instructions. Having the slime-swimming kid be green was also good.
I would love to play an expanded version, so good luck continuing the game!