After delivering Ace nine months ago to join their daughter Maxwell, nearly 2, the star "has been very laid back about motherhood and its had a hugely positive effect on the whole family," a source told Radar.
"Sure, she's still a celebrity and she has assistants and helpers around her at all times so she's not dealing with the nighttime crying, but this new relaxed approach has been heaven-sent."
The source added, "Jessica is totally chilled out and nothing fazes her these days."
The Fashion Star mentor, 33, has been posting Instagram pictures of the adorable tot playing with his former football player father, 34.
Simpson captioned one shot of Johnson, Ace, and Maxwell together, "All you need is love. My perfect humans. :)"
And the Weight Watchers spokeswoman recently announced she'd reached her goal weight.
"She’s feeling incredibly confident, even more so since getting back into tip-top shape, and has fast become the ‘hub’ of the family and mediator/therapist for her troubled parents," said the source.
"She’s not even stressing about her own upcoming wedding!”