Hi JAL,
Welcome to the group. I am probably the bloke you referred to with the 18" scope that lets the kid run riot with the 10"
Congratulations to you on coming to the forum and asking for advice rather than spending a lot of money on something unsuitable. While reputable stores like Bintel and some others will generally give very sound advice, they dont carry all brands so their advice while always be biased towards the products they sell.
The Meade 10" RCX400 telescope your husband has is a fine instrument and is deserving of the highest quality eyepieces, which unfortunately aren't cheap. The scope comes supplied with the S5000 24mm Meade UWA which is a reasonably good eyepiece. There are certainly better eyepieces around this focal length, but they are very expensive and the Meade will do fine.
Steve (Janoskiss) has given you good advice already. I concur with him and would avoid buying any more Meade eyepieces. While they aren't bad eyepieces, there are certainly a lot better eyepieces for the same price or not much more money. I will also add that the Meade barlow you are considering is a 1.25" barlow and will not fit the existing 24mm eyepiece your husband got with the scope. If you get a 2" barlow your husband then has the luxury that it is useable with all eyepieces, both 2"and 1.25".
The scope has a 2000mm focal length. Ideally I think your husband should end with the following eyepiece set or close to it.
1) A low power widefield eyepiece like a 40mm Pentax XW or a 40mm TMB Paragon.
2) The existing S5000 24mm Meade UWA will do for low/medium power. The 26mm Nagler is a lot better but about $800.
3) Something from 12mm to 17mm would give around 120X to 160X and be ideal for medium power views. Consider the 17mm Nagler T4, 16mm Nagler T5, 14mm Pentax XW, 14mm Denkmeier, 13mm Ethos, 13mm Nagler T6, 13mm Vixen LVW, 12mm Nagler T4 and the 12mm Pentax XF.
4) Something of higher power around 9mm or 10mm focal length. Consider the 10mm Pentax XW, 9mm Nagler T6 or the 8.5mm Pentax XF.
5) Lastly, something around 7mm focal length for high power views under good seeing. Consider the 7mm Pentax XW or the 7mm Nagler T6.
My long term list would comprise.
40mm Pentax XW
26mm Nagler T5
13mm Ethos
10mm Pentax XW
7mm Pentax XW
Unfortunately all those add up to a lot of money so you probably don't want to go there at the moment. On the basis you can't afford to get to this list in one go, I would recommend in the short term buying a 17mm Nagler T4 and a 2X Televue Big Barlow. This will cost about $820. This gives your husband an effective 4 eyepiece set of 24mm (83X), 17mm (117X), 12mm (167X) and 8.5mm (234X). There are certainly gaps but this option covers most observing situations reasonably well.
Have you considered selling the kids or a kidney for him and buying the full set of eyepieces I have recommended straight off the bat?
Cheers,
John B