The weird part about them staying over is that their partner isn't also home, since they have to text them this information. That makes me think the idea is they're living together. Also in either case it implies that OP is taking care of the kid, since I don't think you should just leave an eight yr old home alone.
Possibly she texted him so that the kid wouldn't overhear them talking about his gift. My daughter's Dad and I used to do that all the time, especially for private talks that we thought might hurt her feelings.