What do you consider a 'small' budget? For some that might be $25 per kid, for others it might be $250 per kid - so it would depend what you have for your budget. Assuming it's much closer to the first option, these would be my suggestions:
4 year old boy:
1)Some sort of ball (soccer, basketball, baseball) - if you can find it used on craigslit - maybe $3 - $5, if not, maybe $10 (try the classifieds too)
2)Some sort of truck or cars (maybe remote control) - if you can find it used on craigslist - maybe $3 - $5 - if not, CVS or Bon Ton has some in the $7 - $10 range (add in extra $4 or so for the batteries)
3) A stuffed animal - likely someone in your family has a bunch they want to get rid of - ask around and offer $1 and I'm sure you'll find a bunch of takers who want to offload some stuffed animals.
4) Some play doh or crayons and paper or coloring book - Think DOLLAR STORE for these. Likely can do all this for $5 total
5) One other item that he needs (and you would like end up getting anyway) - does he need new sneakers? New pair of PJ's? a new jacket - let's put this one at $12 (but you would be spending anyway)
6) - STOCKING STUFF - think lots of stuff that is very cheap. Again - if you have craigslist or a family or friend that is wanting to offload stuff you could get a ton for cheap - little army men, marbles, animal figurines, chalk, candy, socks - if you come up short on used - try dollar store - total maybe $8
So - that's 5 - 6 gifts plus a stuffed stocking for the range of $32 total if you find a lot used...maybe $45 if you can't find used.
For the dd8 how about these:
Here again, I would try now to find some used things - ask a friend who has a dd a bit older than yours and see if she's wanting to get rid of some stuff - offer to save her the trouble of a garage sale.
1) Littlest Pet Shop - if you can find used set $3 otherwise $12
2) 'pretty' stuff - a couple of nail polishes, brush and/or headband - Dollar store would be good here - $4
3) A CD or DVD - tough to find a deal here, but if it is something she'll really like - might be worth it - $15
4) Make her something personalized and special just from you - take a dollar store bag or journal book or other similar item and using glue and glitter - write her name in fancy letters on it - $2 (assuming you already have some glue around the house)
5) A book or two - try finding a used book store OR a library book sale in your area (or if you find that friend who wants to unload stuff - ask for any books they've outgrown too and offer a couple bucks). $4
6) SOmething that she needs anyway - shoes, jacket, jeans, etc - you would be spending this anyway. Again, try for used maybe $2 otherwise $15
7) Stocking stuff - candy, cute pens/pencils, memo pad, toothbrush, bracelet, socks - dollar store - $10
OK, so low end budget for dd using what I suggest would be $40 high end would be $55.
Not sure if that matches what you're thinking for low budget - but this would work.
Good luck Santa!!!