Hi there, thought I'd weigh in on this
Breachers are great as a counterpunch unit. Because they can have really destructive shooting they can be great for pushing out against entrenched enemy objective holding units. However since your opponent is using orks they're not really the best choice. The extra strength and AP with their 5" profile is kinda wasted, and once you kill 5 or 6 orks with them the other 25 in the mob are going to shoot and chop them to bits. The better choice against horde armies is by far the Strike team. They have the range to stay back, but they keep the killing power against low armour targets. Using a devilfish loaded with Firewarriors and a fireblade would be a good little taskforce to push forward for an objective. They'd need quite a bit of fire support to kill/significantly reduce any nearby ork blobs to ensure they don't just get charged and chopped up, but if you take out a decent chunk of the blob that wants to go for them then the devilfish is a great roadblock. Once the strike team inside has bailed out and claimed the objective, the devilfish can charge into a reduced ork mob and tie them up, protecting your Strike team from getting shot/charged by that unit.
As for your kroot, as most people have said, they are good for screening by being set up as a larger bubble using their vanguard move. This stops any deepstrike or deepstrike equivalent from being able to close with your line. However they're about as tough as wet tissue paper and will die in droves if anything looks at them funny. Against orks this isn't as bad as normal since they don't have the best shooting, but a mob of 30 boys will kill a fair few with weight of shots. If your nephew is a fan of using the psychic power "Da Jump" to get a big mob up in your face, Kroot can be effective for limiting how close he can get to you. Essentially, kroot aren't bad as such, and will be little more than a speedbump, but they could buy you another turn of shooting by virtue of being a speedbump. That's where their use lies in my opinion. 120 points could get you 2 units of 10, which should be enough to bubble wrap your main army block.
Out of curiosity, what is your playstyle? Do you like to form a gunline? Be as mobile as you can? Or a mix of the two with a main army block and a good chunk designed to be mobile. From your desire to use devilfish I'm guessing you lean more towards mobility.
Hope that helps! R'Kai