Madge don't say if you don't want to, but how old (roughly) are they? My ds is 4 and dd is 10. These are good ages generally as in they can do things for themselves but are still dependant on me for stuff. But also there are times they totally drive me mad. If I want to I can find things I like about them and their age, personality etc. It takes time but sometimes positive thinking can make it better. Not lying to yourself necessarily, just noticing things you might not otherwise. Oh one has tidied away a tiny bit of stuff and I only needed to ask twice, that is an improvement etc. And tell them and give a thumbs up. When you get a thumbs up back it is great.
If you say how nice they look or how clever they are you usually (eventually) get compliments back. This builds good feeling.
Are you parenting alone or are you in a couple? If alone, do you yet have support of someone trusted who can look after one while you have time with the other? This is very key. When ds came he was 3 so not at school, so dd (birth child), who had been a lone child for 9 years, only got the tiny bits of time, when ds was in bed, to be alone with me or my dh. We now date night with dd and try once a month for one of us to take her out for an ice cream slush puppy or whatever (me) or a cycle ride (DH - sportier than me!). That works if there is a big age gap, not necessarily for you!
But if you can make some time for them separately, do, to see what they like and develop the appreciation for who they are.
Can I add a few extra things to my earlier list, please .....
Who does bedtime routine (if you are in a couple)?
Bedtime is sometimes the time when a child is scared or clingly or wants to chat etc. These can be great times for bonding.
Because my DH is at work Monday to Friday full time and I was at home (now back at work part-time but still around after school) I allowed/made DH do betimes for the kids 5 out of 7 days (except when on hols). This seemed right and fair. However, I do think bath and bedtime, story time etc can be a very special time and if your other half does it most of the time you miss that. This is not relevant if you are doing it all!
If you can (and they are old enough) built time to talk into the bedtime, just 5 minutes before or after story or before or after bath. For the first six months all ds wanted to talk about was his old foster carer. Which was fine. Some times I got to choose but mostly him. The day he did not want to talk about foster carer was wonderful for me!
Build in the little in jokes and routines, at bed time my son sometimes says 'Night, Night, sleep tight, make sure the bed bugs don't bite, is they do, get a shoe, crack their little heads in two!' Not very nice but he likes it! I think I taught it to him.
If they are TV watching/computer gaming age cut that down slowly and give less if you can. I have found TV cuts communication and makes children's behaviour worse! Who knew!
We all need it and mine watch PLENTY but we are trying to have rules about when it goes on and off and all that stuff because they are nicer when they watch less telly! Easier if they cannot operate the TV themselves!
Outdoors - build a den together in the garden or put up a tent and get close, head to head in a small space might help them to see you more as fun and you to see them as fun - or it could be a massive unmitigated disaster, if so, please do not blame me!
Indoors - I buy cheap little things like matching games from el cheapo shops and although dd at 10 is not interested ds is still very keen to play any game which is age appropriate.
Charity shops sometimes have boards games (and masses of jigsaw puzzles) that start from age 3. Remember the aim is not just to do the puzzle but to do it together and comment when they get the bit in the right place! Very hard with my ds as he moans a lot! if the bits do not stick together he gets very frustrated. Harder card or wood ones are better for us but the very cheap ones he might find frustrating and must be appropriate for the age your child operates at, not the age they are. Sounds obvious but we all forget. Ds is more like 5 and dd more like 8 so in many ways there is 3 rather than 6 years between them!
The nurture game is a well worth investment too at £25 it looks expensive but it builds community in a family really easily and has no working bits or batteries to break! familylinks.org.uk/shop/pgl-shop/the-nurturing-game
Sorry this sounds like a massive brain dump, and it is but hope it helps. I guess the message is you can't build connection in isolation, it happens as you are just doing things. I am sure you know that, but when you feel tired and guilty and fed up (as I do, a lot) it is easy to forget!
Good luck.