"It's very amicable. They're on very friendly terms and speaking – it's fine," a source close to the couple tells PEOPLE. "They've just grown apart. They've been growing apart for quite some time."
"It's all about [their 2-year-old son] Bronx for them. They're fantastic parents – Pete says she's a great mom, and she says he's a great father," says the source. "It's about Bronx's care and well-being and happiness for them."
There was no third party involved in the split, and Simpson-Wentz, 26, and Wentz, 31 – who has been touring with his new band Black Cards – continue to sort through their emotions, according to the source.
"They're both really sad. It's a sad ending to something that was really promising," says the source. "They're each trying to figure out what this transition is going to be like."