Yes, I would suggest those same things. We doubled folic acid from 1 mg to 2 mg. You can also try folate. We've never tried that but some parents say it works better for their kids.
Leucovorin is folinic acid. It is actually used as a "rescue drug" for MTX when MTX is given to cancer patients. Those patients get VERY high doses and it prevents toxicity that would come with MTX. Those doses are in the 1000s I believe - NOTHING like what our kids get.
In small doses, Leucovorin does help with the side effects that come with the MTX doses that our kids take. But because it is a "rescue drug" for MTX it does reduce its efficacy. So it is usually used with higher doses of MTX. Almost always above 20 mg. Often for 25 mg which is the max dose.
We did use it for M when she was on lower doses. We figured even if we lost some efficacy if we got any MTX into her, that would be a huge victory. So we used it at 15 mg and then at 10 mg.
At 10 mg the MTX +Leucovorin did nothing for M. But at 15 mg even with the Leucovorin she felt like the MTX worked. Unfortunately, it still made her sick. But M's side effects were extreme - extreme dizziness so she couldn't get out of bed for 2-3 days per week. Severe nausea that lasted all day for 2-3 days. Vomiting sometimes. Fatigue - extreme fatigue.
Leucovorin is better known by rheumatologists than GIs since they have been using MTX for much longer. But I bet your GP might even be able to prescribe it (though ideally it should go through your GI of course).
If H has severe nausea, we were allowed to give Zofran. One dose before MTX at night. For the next two days, she would get it 2-3 times a day. So if we gave it Friday night, she would generally need 3 doses on Saturday and on Sunday. Usually 1-2 on Monday too.
We also tried Ativan to see if her MTX side effects with anxiety related. If they were, we were going to use CBT to help her deal with them. Ativan just made her sleepy, it didn't help the nausea or vomiting, so they concluded it wasn't all caused by anxiety.
But that anxiety is quite common in kids. Kids to often develop an aversion to the color yellow after being on MTX (the injection and the drinkable solution were bright yellow). They get worked up if they smell an alcohol swab. Things like that. A psychologist could really help.