Your kids are both too young to know how to play alone much yet, especially the 16-month-old, who's barely out of being an infant. And the four-year-old wants to interact with you. Is that really bad? You can teach her to play alone more by doing some playing with her and being fully and happily engaged when you ARE with her, then being firm but kind, and setting her up with something interesting to do, when that timer goes off and you have to move to the next thing.
I'n not clear if the kids are seeing you and DH both mostly at night/after work? Are they in day care? If so, they might be wanting to see you both just because they haven't been with you during the day. Nothing wrong with day care, it just means they are so ready to visit with you and like to see you.
Of course the adults do need down time and time to get things done. One poster said that if you're cooking or doing dishes etc. it's not play time, but I differ. Your four year old is at the perfect age for you to set her up with some pots and pans and plastic spoons in the kitchen and let her "cook" alongside you on the floor, or let her wash the plastic containers once you're done washing the other dishes, etc. Sure, there will be splashes and a wet floor, but you might be surprised how she'll be happy to wipe up water with paper towels if mom's attitude is positive and happy: Wow! Can you be a big girl and wipe up that water? Great job!
Yep, it makes things take longer, but believe me, in a very few years she will not want to play with you, and the chance to interact with her like this will be gone for good.
There's no ideal "good time to devote to playing." It depends on the kids and the parents' needs too. Play time does help you get to know them as people and whether they're artsy, imaginative, analytical, physically outgoing. And once the 16 month old is a bit older, they will keep each other amused much more than they can right now, which will be a big help to you. In short...to me this isn't a bad behavior/discipline/"weak" parents issues. It's normal for kids to want parents' attention and for kids this young to not be able yet to go in a room and be told, "Go play." If you send them off alone, do give them some ideas and set out some toys and get them going.