Speak kindly should have a time limit on it - so either per day or if that's too hard, break up the day into morning/afternoon/evening. Then it's quantifiable.
I would use "Speak respectfully" rather than speak kindly. You don't need him to always be kind but you do need him to always be respectful. Kind isn't always the right response either.
Does the unkindness always involve words or is it also physical, winding up etc?
But anyway by wording it as being respectful it's clearer what you want. It also helps to sit down and think about what him being respectful would look like - if you have to start with examples of what not to do, start there (you do this privately) and turn it around and write down how you would like him to handle that situation. E.g. DS is annoyed with DD for coming into his room. Wrong: Shove her out, shout at her to get lost. Right: Ask her politely to leave/place a note on his door to explain he doesn't want to be disturbed. E.g. DS is bored. Wrong: DS winds DD up for a bit of sport/fun/entertainment. Right: DS asks DD if she would like to play a game / asks mum/dad if they can do something with him / takes an idea from the ideas basket.
You need to know what the "right" answers look like because then you'll notice if and when they happen and you can either give a sticker on the spot, or just say "Well done DS, that was a really good idea to write that note for your door". Conversely, if you see something non-respectful starting to happen, you can give a short warning in the form of "That doesn't sound very respectful". You're not wading in and telling him off so he doesn't get immediately defensive, but it gives him a chance to remember the goals he agreed to and change direction. If he does then backtrack and manages to be respectful on that occasion it wouldn't represent a loss of the point for that time slot. OTOH if you are too late and already walk in on him being nasty then you can separate them but also say "That wasn't very respectful, was it?" It takes the fight out of the moment, because the penalty (losing the point) comes later, but it still enables you to give that reminder. Obviously you still need to physically stop the behaviour by separating them or whatever is appropriate, but try to curb the urge to tell him off about it at the time.
Be careful of overloading the system by having too many things on it. If they get a reward for every little thing then they don't have to try very hard on the things they find especially difficult. I would come up with maybe 5 points to work on. If they have different points to work on each, max 3 for each child. Otherwise you'll never keep track of it and it will get dropped very quickly.
I think I would do it as points or something which they can spend on the reward of their choice. The reward shop opens periodically, and they can buy whatever they like. If you want, enable them to pool their money to get something better! Then you can have things like a magazine, sweets, small toy (e.g. single Matchbox car out of a multipack, accessory for collectible doll etc) but you can also have stuff which is free for you or which you'd probably spend money on anyway, like "Choose what we eat for dinner" or "Family movie night" or "Swimming trip". This is called a token economy and is a good tool because it's not only motivating for behaviour, it's also helpful practice in skills like saving, sharing etc. You need to work out what is the max number of points they could possibly get in a week in terms of "pricing" the prizes, but bear in mind that as it's a work in progress they aren't necessarily meant to get every single point every day/week!
For praise I find "Thank you for..." rather than "Well done for..." is much more authentic and doesn't sound wrong for a child to say to another, which is actually good, if they are modelling it!
Don't be afraid to involve the kids in the process a bit - if they are involved in setting the goals they are much more motivated to work towards them.