My wife had a conference in Putrajaya (Kuala Lumpur, Malaysia) recently, and would have been gone for a few days.
When she had a similar conference last year (in another city), I took on the task of single parenting my daughter (two and a half years old) for a few days.
She was fine the morning my wife left.
She was fine that evening.
But when it was dark and she was supposed to go to bed, she started to tear, and said that she missed her mommy and finally sobbed herself to sleep.
I got my wife to video call us while she was there, so my daughter could see her. And it was clear she really liked being able to see and talk to mommy.
I got my wife to send video messages via WhatsApp and that was precious to my daughter.
But bed time was still sad times. And lonely times. And times she missed her mommy acutely.