My two were the same age when they were placed with me and at one week in I thought I was losing my mind. It’s very very hard, at the moment they have literally lost everything and everyone familiar to them and will be completely terrified. As others have said, it’ll take a long while before they will feel confident or settled - their world has been turned upside down as has yours.
I’m not sure what you mean by them testing the discipline you’ve put in place, at this stage it’s all about building relationships with them, helping to calm their fears and find their place in the family. They won’t know where things are, what belongs to them, they’ll be grieving the people and things they’ve left behind - the more you can do to help them settle the better.
All of this, of course, at a time when your world is on its head too - when you’ve got everything you wished for and it’s massively harder than you could have imagined and when people are telling you to “enjoy your kids”, while you’re actually going through some kind of exquisite torment at the hands of these two littles.
What helped the kids were clear routines and plans for the day, repeated often in a “so now we’re having breakfast and then we’re going to get dressed”, “now we’re getting dressed to go to the park” kind of way. Visuals can help too. Giving them familiar food - even if that means chocolate cereal or fish fingers, don’t try introducing yet more new things so early on. The first thing we really worked on was a good bedtime routine, going to bed much earlier than the foster carers had done - we would be very active during the day and settled in bed by 7.00, early in placement they would sleep for 12/13 hours - the sleep helps them process all they’ve been through.
Someone on here said “don’t try to control what goes into them or what comes out”, feeding and toiletting are two ways for kids to exercise some control - try not to get angry with them they are trying to meet their need for control. I say this as someone who went through 17 changes of clothes in one day!
What helped me was having the day broken up so we were never spending too long on one thing, my DH would take over and give me a break when back from work - sometimes he walked in and I literally walked out, got in the car and drove... just getting some physical space helps.
Expect everything to take 5 times as long to do than you think it will. Get a cleaner, do online shopping accept practical help from family and friends. I also saw a therapist, which gave me space to process the enormous changes in my life and how hard I was finding it - she was worth her weight in gold.
Spend time with good friends, who will listen, not judge and understand that you’re having a hard time. I remember meeting a friend for coffee and just crying for 2 hours straight. Use the social work visits to be honest about what you’re struggling with - both my and the kids social workers were fantastic supports in the early days, again no judgement but lots of understanding.
The early days are so very hard, go easy on yourself, and on them - it gets easier but honestly I’d say it took a good 6 months before I felt comfortable in my own skin again.