When I was 3 I was terrified of the problem blob, which caused my 3 Yr old self to hate the show from then on. now a decade later I'm thinking 'Lol I was such a wuss'
For rewatchablity, I'd say it's pretty good, even if you are older than 5, besides it is extremely nice background noise for when I'm drawing or animating, definitely would recommend that you watch some episodes on the YouTube Channel they have, if they do get scared, just tell them that the characters don't exist which mean they can't hurt you.
Another random fact, I do not remember watching any episode where the number taker was ever featured as a 3 Yr old, now I wish I did actually watch an episode with this guy, because him, along with The Shape Japer, are my favorite Meanies in the series.
My thoughts on the numberjacks in general though (In terms of personality, here they are.
Zero, One and Two are by far the best out of them all, 3 is a little bit of an annoying character, 4 and six are a little generic. 5, to me, is a little bit of a jerk...but also a bit of a savage. 7 8 and 9 are pretty nice to learn from.
Q0/10 loved the childhood memories and nightmares.