I dunno. Apparently Trunks telling him he was going to do Chi-blasts appropriately wasn't enough, or he probably wouldn't have felt a need to remind him.
I don't think the kids are close to the adults.
If Trunks saying "It's #18" is enough to negate his previous comment, then there's no reason for him to take Goten's suggestion on not using full-power into consideration. Trunks says, "I know, I know", so not using full-power was something that likely would've took place whether Goten reminded him or not.
I work with it by looking at the context in its entirety.
Trunks wants to use Chi-blasts because they can't fight in the costume. Goten isn't so sure about the idea. Trunks reassures Goten that she won't die if they do it appropriately. He then mentions that she's #18, implying she's tough. That means she was still tough enough to take Chi-blasts (without being killed) in general because he still intended to finish the battle with one.
That doesn't mean she can take full-power Chi-blasts. #18 is tough enough to take Chi-blasts done appropriately. #18 is not tough enough to take any Chi-blasts at full-power--hence Trunks deciding to hold-back.
Trunks intended to take her down with a Chi-blast. Just not a full-power one because it wouldn't be appropriate in the situation.
In Trunks' mind, his held-back Chi-blast would've been enough to win the fight without them needing to engage her in a close battle. So from his perspective, Held-back blast>#18.
I took it into consideration and I think my explanation above gives a good solution for it. If it's enough to negate what he said in the same sentence, then him opting not to use full-power means he wasn't comfortable with hitting her with his strongest attack. The only reason why he wouldn't be comfortable is if he thought she couldn't handle it. That means the part of his dialogue about needing to do it appropriately is still pretty accurate.